So I was wondering what's the normal shelf lifr of home brew is. I normally store them at room temperature in kegs that I force carbonate. How long does it take till there not drinkable anymore?
As others have said, it depends on the style.
The beer killers are temperature and oxygen levels. Homebrew is going to have much higher levels of dissolved oxygen than properly made commercial products (less than 0.1 ppm dissolved oxygen). Under room temperature storage, pro-brew is going to start showing oxidative notes within 100 days, so you can assume that your brew is likely going to lose condition faster than that.
That brings in the other big factor: temperature. If you can keep your beer at freezing or near freezing temperatures (32-39 F) then storage stability will be massively improved. Pro-brew kept at those temperatures can remain stable for up to 3 years.* Cold storage also slows down rates of microbial infection, if you're afraid that a beer might go bad.
So, if you can, get a refrigerator for your kegs!
* Data from C. Bamforth, Scientific Principles of Malting and Brewing.