[vespine]

Originally Posted by matt

Based on that, how much time should I get out of a fully charged battery with very moderate use of go-to and setting the controller on low-med?

That's a tough one, unless someone has specific experience with exactly this set up and viewing habits it would be hard to guess, even roughly. My guess tho is that the motors of the go-to would be the thing that use the most power so the less you use that the longer it will last.
Another thing you can do is calculate how much power the heater uses, I've googled it but can't find what the wattage of it is, do you have the specs? Looking at other home made dew heaters it looks like you may need as little as 1 - 4 watts, say it's 3 watts, 3w/12v = 0.25 amps per hour. Which very roughly means your battery will theoretically run a 3 watt heater for 68 hours Now, that's 'theoretical' which doesn't account for imperfections like discharge rates, internal battery resistance and other "inefficiencies", in reality it will probably be considerably shorter then that, around 80% for lead acid batteries, which is still over 50 hours.
On the other hand, small electric motors are more like in the order of 30 watts and there's probably 2 in a go-to right? So that's using a lot more power, does the go-to use power while it's "not going to"? If it has a display and stuff it probably is but that would be quite low too I guess.
Over all, I think that set up sounds pretty good really, anything else I could imagine would start getting more complicated and much heavier. My guess is that the battery should last at least 15 hours with the type of usage you describe, but ultimately you'd have to try it, could be a bit less, could be a lot more. That sounds like a couple of nights viewing between charges. Lead acid batteries, unlike some others, are generally not adversely affected by frequent recharging so you can charge it up every night after use if you want.