How Long to Use Heat Mat?

I've been using a heat mat to start my seeds but they are all small plants now (about 2 1/2 months old). I've run into problems where they looked fine one morning and then the next morning they are lying on their sides with the leaves all shriveled up. Most came back after a good soaking but I've lost a few that never revive.

The temp around the root ball (Jiffy plugs) with the heat mat is 80F. If I turn it off for a few hours it goes down to 60F during the day. I don't know what it would be over night but I would expect cooler.

Do I still need to keep the plants on the heat mat or are they old enough to do away with it now?
Do you mean the little Jiffy peat pellets? If so, at 10 weeks your plants are too large for the plugs and need to be transplanted into something larger.

As to the heat mat, I leave mine on until I move them outside for hardening off. I just put the first set out in the cold frame this morning.