$$ to run heater

I was wondering if anyone had a general rule of thumb about how many 
hours per day a heater is on.  In order to calculate how much $$ I'm 
spending to run various instruments in my aquarium I need to know how 
long they're running.  This calculation is easy with lights and pumps 
but I can only guess as to how long the heater runs.  Specifically, in 
my 50G tank which I keep at 75 degrees in a 70 degree room, I'm guessing 
that the heater is on 4 hours / day.  Does this sound conservative or 
liberal?  Given the number of variables to consider I imagine this would 
be a tough one to nail down but what are people's opinions?


					Miles Morrissey
					Easthampton, MA