We have a tiny, old water heater, I can never even shower and shave my legs at the same time without it running out. I had a fishy pool with DS, and knew it would take several tanks to full it. I had filled a AquaDoula with DD2 (in a different house) over a couple hour period while in labor and figured it would be similar.
When labor hit with DS though, I was dying. I had bad back labor, and I needed in my tub soon. We filled it up with the first tank of water, which wasn't that much, and I had Dh started putting every pot we own on the stove, we have several huge stock pots. The water level was low I could sit in the bottom, and lean over and get a portion of my belly in the water, so that is what I did while yelling at DH between contractions to boil more water.

I am guessing that it took an hour to get the tub filled up, probably a little more, and it stayed hot for my 4 hour labor, we never added more water. And yes, we had the tank turned up at high as it could go. DH would pour in hot water and then add cold until the temp was right and we just kept doing that until there was enough water.