[Dominic]

All light bulb designs have a model number, if you stick it in google, you shuld be able to find the life in hours.
My Snake uses B22s, and they are rated for 1000 hours, 6weeks 24/7 or 12 weeks 12 hours.  I would guess you have Par38s, which have a 2000 hour average life.

You can get some rated for 10,000 hours, but I dont think they come is sizes suitable for 6 chickens.

[Squiggly]

By using a heat mat designed for reptiles and then buying a thermostat aren't you hitting the cost of an electric hen??

Or am I missing something (which is highly likely knowing me!)

[Dominic]

Probably more expensive actualy.
I just already had a thermostat for the snake, and a heat mat for that matter.