When should I remove the tape?

The biggest mistake I see when people are using tape is this... they tend to leave it on for way too long.

The ideal time to remove tape is immediately after you've applied your last coat. The problem is that if you leave the paint dry for too long, the paint film will have had time to dry and create a continuous film going from the wall and overlapping onto the tape... this means that you're essentially going to be tearing this paint film in two as you attempt to remove it. But by removing it as soon as you finish your last coat the tape will come off much easier and won't peel off the paint.