Early on, when Piper first got her powers, she freeze thing as a reflex to her fear, if not on purpose, but the things or people would always unfreeze after she did what she needed to do with them. Also, they would remain frozen for different amounts of time but each time they’d unfreeze after Piper didn’t need them to be frozen anymore. Essentially, once the goal Piper had in mind when she froze a person was attained, they would unfroze even if it was at a bad/awkward moment. I honestly think this fits in with the whole “magic runs out when it runs it course” rule that popped up every once & while.

EXAMPLES: 1. In EP1 she froze Chef Moore because he was going to taste her food without the last ingredient. As soon the final ingredient was added, he unfroze. 2. In S1, she freezes Josh (I think it was him) while they’re on a date to look at his wallet. As she’s putting it back, and he reason for freezing him expired, he unfroze. 3. Also in Ep1, she freeze Jeremy to get away from him. Once she was away from him, he unfroze. 4. When she met with Victor in S1, Piper froze the room to stop the waiter from dropping that dessert but when grabbed it the room unfroze. 5. I can even remember a time she froze a falling box of cereal which unfroze immediately after she put a trash can under it.