At the moment i have a 40 gallon.

How long do i have until i would need a bigger tank so it woudlnt restrict my oscars growth or harm him?

you need to upgrade now, unfortunately you haven't got time on your hands when keeping Oscars. They are not like clown loach where you could keep three or four of them in a 20 gallon for a couple of years before you needed to upgrade. Oscars can easily reach nine inches in their first year of life, they grow to around 1 inch a month. It takes around six weeks to cycle a tank, even then, it's not ready just to take a large fish. I would advise anyone who wants to keep Oscars to not mess around with little tanks, get the tank you intend on keeping the Oscar in for its life first, it'll save you a lot of hassle in the long run

Ok if you have just 1 oscars on its own you could just get way with 40g. But A little bigger would have been better. most people want more than 1 fish, Your oscar can reach 6" in 6months or a little bigger. and upto 14" in perfect conditions. most fish will reach 9" or 12" in a tank within a yr. the main problems are a fish this size can make a lot of mess and it becomes a fight between you changing the water weekly and the fish doing his best to make it dirty again. For your fish to be happy and to make you life fairly livable, 1 fish to 40g will work. he will grow fine and you will only need to change one 3rd of the water a week but will need to test wky too. you will also need a very good filter. for a tank of 40g i would say 120g filter or more. So thats one option.... If you want more fish, as you already know a bigger tank will be needed as soon as you start to add more fish or your work load will mean 2 or 3 water changes a wk. this will become too much work for you and at best you lose interest in your fish. you really need to decide how many fish you want to keep and buy a tank for this. If you are going to buy a new tank and it is oscars you want to keep, get the biggest you can afford and have room for. this will mean your work load is less(less water changes) or you can add a few more fish. If the size of your tank is limited to cost or room, so will the amount of the fish you can keep. if you already have other fish and depending on their size you could need a tank very soon. testing your water for ammonia,nitrites,nitrates will tell you this, and this is why most people ask for water tests before helping. For your fish to grow well and be healthy all you really need to do is keep the water in the right conditions for that fish.So 1 oscar in a 200g tank will mean you could get away with one 1/3water change a month. put 4 oscars in and again your back to wkly changes.

personally i would get a bigger tank asap. perhaps in the 90 gallon range. this will not only give your oscar more room to grow but will also lighten your maintenance burden and allow for a couple smaller tankmates. if you do upgrade a good rule of thumb is to have filtration that is double your tank size.

as ofl said it takes about 6 weeks to properly cycle a tank and in that time your oscar could grow a couple of inches so time is indeed limited in your case

Yeah at the moment the limiting factor is space in my house.

Unless i throw away the piano, a 4ft tank would be in the middle of my lounge.

For my 40g tank i have a Ehiem 160g filter on it.