Hi. I’m Marta from Morristown NJ. Just joined.
At a minimum until most hatch. After that it is really up to you. If you need the mat for more seeds, if there isnt enough light where the mat is, then you'd move them quickly. But I have never seen a heating mat to anything other than speed up the seedling's growth.
Keep in mind our house is old and the windows leak. Normal room temperature is often like 60 degrees during the winter. So any soil warming is probably good here. Its funny to note when I describe my cloning set up, folk ask me why the water heaters.
I think heat at the roots plays a huge role in a plant's health. More so when it is a seedling.