All, thanks for all the comments, even the rude comments about 'how we let it run out in the first place without noticing'.
We filled them both on a thursday, by mid saturday night, one tank was out. I know that the arrow has to point at the one you are using...and that is needs to be open...which it was. Went sunday to Lowe's and picked up an electric fan tower for the bedroom, and a smaller fan for the kitchen area, and the den is covered by the electric fireplace. So, hoping we won't need to use the propane for the furnace anymore. Also, that Friday was the one time Houston got snow, so it was pretty cold. Also hard to keep a 40' FW warm on just the fireplace, and I can imagine that it (obviously) uses a lot of propane to keep it at a steady 65-68 throughout the night and day when it is overcast, wet, and cold. So now I can gauge a bit how long the propane lasts since this was our first experience...
Thanks to those comments who were actually trying to be helpful and not condescending!