Depends on what sort of breeder you are using. I personally recommend a 5 gallon tank (plastic container is fine) with a bubbler, heater and some fake plants. When using this setup, if nothing happens you can leave her there for about a week provided you either use a cycled filter or change all the water every second day. (50% daily).
I'm not confident using a breeder trap with any species other than guppies. I came on now to brag about my young female's first drop (20 live fry, one taildragger, from an inch long fish, first drop!) and because she was so small, using a 1 gallon breeder was not uncomfortable for her. However, I had her in an isolation tank in the trap (so there were no inquisitive fish looking at her, crowding against the walls and generally bothering her) and she is very small, as I said, about an inch long. She was only in there for a few hours because I knew when she was going to drop. But I've had platies and mollies abort fry or even die in traps due to stress.
If you've got a breeding trap or net I'd put the platy in there and watch her closely. If she looks stressed, let her out. You can keep them in there for up to 48 hours but I don't recommend it. If putting her in the trap feed the other fish at the other end of the tank to get them out of the way, then turn off the lights.
