I don't have the same equipment as you do, but I believe many different heaters have similar experiences.
In my Fiber Stream
, the longest I've run the heater without recharging the battery was over 3 consecutive days. I did not run the heater in the middle of the day, only in the evening and overnight through to morning. I left the roof vent cracked open slightly and the small window in my door cracked open slightly.
After 3 days, my battery was down to about 11.6 volts, having started the weekend at 13.4 volts. That's when my camping time was over, and I went home and recharged my battery.
I hope this helps. Someone with a Scamp
will come along soon and add to this...