How long can Sonic stay underwater?
I timed it in my game, and it appears to also be 30 seconds.
I started a stop watch on my phone the second I jumped in, and got 29.90 seconds when I died. The .1 of a second is almost certainly just due to my timing. The countdown bubbles next to Sonic's head go down at a rate of about 2 seconds each from the looks of it. So, once you see those, you have about 10 seconds to get some air.