How ong is too long for the wires as the thermostst is on a wall across the room so it goes from the fire place up aprox 8 ft then across ceiling aprox 12 ft the down 3 ft
Give or take
The package for the thermostat says 750 milli volt
And I have hooked it up as per the #18 post

I would say you are okay on length, only 23 ft. The only way to be sure would be to contact the manufacturer for specs. Have you tried jumping the top and botton post? This will tell you if you have a problem in unit, or in thermostat and wiring. What style thermostat do you have. You may need to adjust some settings for it to work with millivolt. Please try the jump test and let me know what you find.

One more question. Did you use actuall thermostat wire, or other wire. Millivolt systems are very sensitive when it comes to correct thermostats and wiring.