How long a lead may be used to connect a component to a circuit?
As I understand it you want to use this as a presence simulator to keep burglars out, right?  

The thermistor is not a good idea: the bulb will reach its steady temperature within a second or so, then it will only light very briefly. Better use a timer to switch the lights on for a certain (variable) time. You can use 555 timers to create switching events and on-times, but I would use a microcontroller for that; that will allow you to create any switching pattern, even replicate actual switching it recorded previously from real use (though you'll need extra hardware to detect switching events).  

A wire length of 12 feet/3.5 m is not problematic. It may pick up some noise, but I don't think high accuracy is a requirement. The noise will be less if the impedance is lower. For instance you can use a 10 k&Omega; thermistor in series with a 10 k&Omega; resistor, or a 1 k&Omega; NTC in series with 1 k&Omega;, both will give you the same output. But the latter has an impedance of only 500 &Omega; and the former 5 k&Omega;.