heat mats- how long do seedlings need heat?

[dice]

My seedlings are so fragile I wasn't sure if something was wrong or if they would get tougher.

Some people put a fan on them, so the stems will thicken up.
Others just brush them with their fingers a couple of times
a day if the plants are easily accessible for that.

See
http://en.wikipedia.org/wiki/Thigmomorphogenesis

Some big greenhouses have fans for ventilation, but the industy
has chemicals for producing that look in a seedling, too.

Plant them deep, so that just the growing tip and one set
of true leaves is showing. You probably have warm soil there,
so deep planting (deeper root system, so access to water
down deeper) is probably more useful than trench planting
(shallower root system). Then it does not matter if the stems
are spindly at transplant.

It seems like they should already be outside and growing
in the sun in your part of CA.