If Sonic stays underwater for too long without surfacing or finding an air bubble, "danger" music will start playing alongside a countdown leading to Sonic running out of air and dying.
According to an IGN guide, Sonic could stay underwater for 30 seconds in Sonic 2, but I'm unsure whether this applies to Sonic Mania. (Or any other Sonic games for that matter)
How long can Sonic stay underwater before running out of air?
Best Answer
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.