I have tried out a hot bed in my poly tunnel this winter and so far, apart from the extreme cold we had in January it’s worked well. When is the best time to remove it? It’s starting to “run down” a bit and is about 30 degrees C at the moment.
You can rejuvenate them with more fresh stable manure on top. We do that for the basil now sown, then all the cucurbits and tomato plants. So it depends what plants you still may or may not need warmth for.