It depends on the star and your speed. The faster you go, the faster you can escape the star, ie stop the heat rising. M stars are easy, class A and up better wait a bit. You can always abort the fsd right before it hits 100% heat, there is no penalty and the heat will immediately stop rising. Then the trick is to always steer along the correct side of the star so that the destination is never obscured and you can reach it before the fsd is fully charged (Anaconda turns so slow!) It's a fun mini game to keep going. But I did have 2 emergency stops since departure. I haven't had orbit lines on since I found out you can turn them off a few weeks after launch. Once I got a little too close, the other time my brain had a short and instead of pushing away, I pulled right into the star at full speed :/