When to remove from heating mat

Is there a general rule when I should remove my seedlings from the heat mat? I have one flat of impatiens and two flats of petunias. A good majority of them have sprouted (I will try to attach a picture) but I am not sure when I should turn off the heat mat.

I tried this method with some Hibiscus, Lobelia, and Gomphrena. They all came up really well under the lights and heat mat. When they got to about 1 inch in height I took them off of the heat. All three flats died within a week.

I'm not sure what to do. Any advice?

Thanks!

Once they've germinated, they don't need the heat anymore (unless you've got them in a really cold place like your garage). And if you leave the heat on for too long after they've germinated then they will tend to get leggy, so I typically turn it off when I think most of my seeds have germinated. (And I doubt that removing the heat had anything to do with your other seedlings dying a week later--chances are that was due to damping off or something else)

I agree with ecrane. I take 'em off the heat when most are germinated.

After looking at your picture, it appears that your soil is very wet. It even looks like there may be some algae growth in the middle flat towards the rear of the picture. Many people keep the soil way to wet for what is needed for germination and growth. Damping off may be the cause of your problems. When I mix up media for seeding, I only add water until I can just squeeze it into a clump in my fist. If you can squeeze any water from the mix, it is too wet. Also, if your hands feel "wet" instead of just "damp" when squeezing mix, you probably have too much water. Try adding some air flow to help dry out your flats. Remember, it's better to be on the dry side - you can always add more water, but it is difficult to dry out when it's saturated.

I was amazed at the improvement in the health of all my greenhouse plants when I put in
a fan. I have with several smaller fans strapped above my benches and it runs 24/7. Not
only are the plants/seedlings healthier but I have significantly fewer problems with bugs.
No fungus gnats, no whiteflies or aphids... amazing how helpful the airflow is.

Thanks for the great tips. I will add a fan to see how it turns out, I never thought of that!