It's impossible to say, as a lot has to do with your particular set-up. The most important factor is the length of the derailleur hanger. They vary and depending on what you have you may get more or less than 30T. I've seen derailleur capacity affected by up to 4T depending on the hanger. The other factor is the length of the axle slot in the dropout and where you position your wheel. A long slot and running the wheel at the back will often allow you to run extra teeth. The final factor is proper adjustment of the B-screw which affects the clearance to the large cog.