Okay, so up until 2 months ago I would clean my filter by washing in tap water, or getting new media. Since then I haven't cleaned my filter because I learned that I need to keep BB in there.

At what point should I clean my filters? In tank water of course.

What type of filter have you got? Basically, you should clean your media when it gets dirty. There's nothing wrong with a little bit of dirt, but if you start getting a buildup of uneaten food, fish poo, and basically yuk, it's time to clean the media carefully. You are quite correct, you must not under any circumstances clean your media in tap water, tap water contains chemicals that will kill your beneficial bacteria.

Spongers will become clogged up with crap quite quickly, so just submerge them in a bucket of existing tank water and squeeze them out until there is no more horrible debris in the spongers, then get them back in the filter and do not let them dry out. Other types of media that you often find in canister filters can be spilled out in existing tank water. I use one of those kitchen Appliances that you tip vegetables into to drain the water off, I find these excellent for cleaning media.

It's impossible to say exactly how often you should clean your filters, it depends on lots of factors, how big your tank is, how many fish you have got, how often you feed etc etc. Hopefully I have given you an idea of how to go about it. However you do it, try and do it as quickly as possible. Bacteria will start dying as soon as they are starved of oxygen

I have a Fluval 304 and a HOB aqua clear 110. Since I plan on finally getting my 100gal filled should I not bother cleaning the filter for a week or so? Or does it just depend on the 'muck' build up?

Unless your filter is particularly dirty then there is no need to clean it. Canister filters will suck up anything that is in your tank, whether that be uneaten food or fish poo. If you leave it in your filter to build up, then this could actually have a negative effect and start affecting your water quality. Use your discretion to when you should clean your filters. If they don't look overly dirty than leave them, if there is lots of horrible crap in all the trays then give them a clean.

Alright well my nitrates, nitrite and ammonia are all in check so. I'll take look at it when I move my tank.

Just remember to nurture and look after your filter very carefully. The inside of your filter is what keeps your tank alive and healthy, without your filter, your tank is just a reciprocal full of water that cannot sustain life

Yeah I have come to realize that. Mostly due to this site. For 4 years or so I would just clean all the pads and media in tap water, trying to get them to look like new. I did not lose many fish though but I think it would have helped them.

with my new DIY filtering system, i clean the catcher every other day while the
polishing section once a month. i found out that my water remains in perfect
condition even with minimal water change.

in a canister type filter, it is very laborious to dismantle and reassemble
it, thus, you will feel uneasy to frequently do the cleaning it. the only
gauge is when the flow had reduce which means it is starting to clogged up.