Time after germination in a heated propagator

My 'early' tomatoes were not germinating after 4 weeks so I got a heated propagator (£10 B&Q, actually made by Sankey) and they germinated over night. Wonderful, but how long do they stay in there for? They seem to have shot up. Also, I've got some melon seeds which need about 27degC to germinate but I don't know quite how I will achieve that. Any ideas?

Thank you all!

 

As soon as my seeds germinate I get the little "vent" open, and then after a day or so I take the cover off, and after another day or so I get them off the heat.

If they are on the windowsill I put a mirror or a box cut in half and lined with tinfoil to reflect more light onto them.

If they are getting tall and "leggy" they have too much heat, and not enough light.

Tomatoes and Melons etc. need a minimum of 10C (to keep growing, they can survive a bit lower than that, but they will "stall") - so don't stick them anywhere too cold