Hi Danndy! FWIW, I have a 400 gallon tub, 7 1/2' square. I use it daily/every other day. On occasion it gets used twice the same day. We probably average about 20 - 30 minutes per use, plus the time it's open to chlorinate. My water level drops 1" - 1 1/2" inches per week.
First couple weeks I thought it was leaking but it's just evaporation. Hopefully that's the case with yours. Yokeman is right about also losing water when you exit the tub. That amount is greater if you wear a suit (holds water like a sponge.)
BTW, I have a friend in Michigan who just bought a Siberian. I got to see it last week and it's a great tub. . .Congrats!