It’s a 55gal for my Oscar Ronan. How long should it cycle? Produces? Filters? Should I use starter fish? I have never set up a tank this big and I need some pointers!

there are some topics posted here earlier
dealing for that. nevertheless, you can start
importing some water from your existing tank will
expedite system cycle. filtering system must be able to
cycle your tank at least one full circulation in one hour.
this means at least 250LPH or 55GPH capacity. experts suggest
at least 3 to 4 times your tank size.

importing some filtering media from the existing one will also
help to expedite colonization of good bacteria to the new one.
just make sure that you do not have problem with the existing
otherwise you will just be transferring the problem.

Hi I'm going to give you easy to follow instructions, based on my own experience in cycling fishless 8 tanks so far, from 1USgal to 145USgal.

What do you need for fishless cycling a new tank?
An inside the tank glass thermometer (LFS)
A bottle of ammonia solution without additives or surfactants. (supermarket, household cleaners section)
A plastic syringe (no needle required) or teaspoon measure
A Freshwater, liquid reagent based, test kit for: Ammonia, Nitrites, Nitrates and pH (better if you have GH and KH too)

Note: to make sure your ammonia solution has no additives shake the bottle and see how bubbles behave; if it looks as shaking drinking spring water it's safe, otherwise look for one that does.

Tank set-up:
Secure your stand or use 1/2" thick cork sheet if placing at floor level.
Place your tank at the desired location. Check it's reasonably leveled.
Place the substrate you will use (gravel, sand - you should have rinsed it previously)
Fill your tank with tap water (Ok to use a hose)
Treat water with proper amount of water conditioner (use only those that remove chlorine/chloramines and heavy metals) make sure it doesn't lock ammonia up.
Place the filter(s), remember to rinse activated charcoal in treated water before putting it in the filter. Turn your filter(s) on.
Place your heater. Turn your heater up(84F)
Place your airpump, check valve, airstone(s). Turn on your airpump.
Place your canopy and lights. Turn on your lights.

Let your tank run for at least 30 minutes so you check everything is running well. Look for leaks. Listen to your filter(s) and airpump. Check the pilot light on your heater. Check the water temp.

Take sample of your tank's water and register your readings for:
pH, Ammonia, Nitrites, Nitrates.

Fishless Cycling with ammonia solution:
Actual dosage of ammonia solution you will need depends on the concentration of your ammonia solution. This is the only hard working part. You will need to make several measures to get where you need to be safely, erring on the lower side is desirable. Our goal is to reach the range of 4 to 6 ppm. Aim at the lower end. If you go beyond 6ppm, you will need to do a partial water change to bring it down to safe range.

Unless you get one solution with unusual strength, add 5ml of ammonia solution to your 55gal tank. Wait 15 minutes. Take a sample and measure for ammonia. Say you achieve 2ppm with that (just an example)

Write down: 5ml = 2ppm.

Add another 5ml, wait for 15 minutes, measure again. Maybe we were expecting 4ppm, but the reading says 5ppm.

Write down 10ml = 5ppm. Do no more. Next day you use enough for 4ppm, say 8ml = 4ppm.

If otherwise it reads less than expected, say 10ml = 3ppm, add 2.5ml, wait for 15 minutes, measure. You should be in range. If it reads 4ppm, you add 12.5ml daily. I guess you got the idea, there will be errors of measurement, but if you handle it safely, your tank will be cycled in 2 weeks tops!

Fishless cycling with ammonia solution works well if you feed your tank within the 4 to 6ppm daily, aiming at the lower end of 4ppm. To speed it up you keep the tank temperature high (84F), provide strong water surface agitation and use airstone(s); also keep the lights on, specially if you are cycling with plants (more oxygen goes into the water).

Now you leave your tank on 24/7.

Keep measuring for Ammonia daily, before you add your daily dosage, also measure 15 min after you add the solution, at least the first two or three days since at 8ppm your cycle will likely slow down due to too much ammonia.

Feel free to adjust your daily feedings of ammonia solution so you remain in the 4 to 6ppm range. If your bottle of ammonia solution doesn't close tightly, concentration may vary due to evaporation.

You will not do any water change no matter how ugly your tank looks! bacterial bloom is often confused with algae bloom.

In 4 to 5 days, you will notice your ammonia readings before adding the daily dosage dropping to zero or close to zero. At this point (before adding your daily dosage of ammonia solution) measure for nitrites. If you have readings below 1.0ppm for nitrites, add the same amount of ammonia solution to feed your tank as you did the day before.

Next day measure for ammonia and nitrites. If your nitrites are equal or greater than 1.0ppm, you are in the second phase of cycling, and from now on, you will feed your tank only with half the amount you were feeding before.

Now this enters in suspense mode. The Nitrite spike last more than the Ammonia spike. However, if you do this carefully, in 8 or 9 days your tank will be ready. It may look like forever, but the Nitrite spike will fade down. 0.5ppm, 0.25ppm -keep on feeding!- once you read zero nitrites you are done!

Turn off your heater. Do as huge water change as you want, add treated water, Turn on and adjust the heater to your desired temp. Measure pH, Ammonia, Nitrites and Nitrates. Add your fish, buckle up, enjoy the ride.

I know this is long, but English is my second language. Hope you can go this way, this method has worked wonderfully for me.

Pepe
Santo Domingo

since pepe covered the cycling part of your question i will only mention that if you are planning on using a hob type filtration you should aim for as close to ten time your tank volume per hour in total flow.(e.g. if you use an aquaclear brand filter you should look for one that will approximatly 550 gallons per hour. like an aquaclear 110 model)
if you plan on using a canister type filter you should aim for about 5 times you tank volume per hour.(if you use a fluval something in the 405 model range.)
this will ensure you have enough filtration for your oscar now and when he is mature.

and remember you can never over filter an oscar tank.