I'm Guessing on these volumes : Pool 22 x42 = approx 31,000 gallons Spa 7' round = 600 gallons
you can calculate the approximate amount of time it will take to heat the spa alone by the following:
Gallons of water x 8.3 (lbs/gallon) = X lbs. of water
X lbs of water x your temperature increase = Y BTUs required
Y BTUs required / BTU output of your heater = Z Run Time in hrs.
Example:
600 gallon spa x 8.33 = 4998 lbs of water
4998 x 50 (50F to 100F = 50 degrees in increase) = 249,900 BTUs required
249900 / 158000 BTUs = 2.5 hrs to get it up to your desired temp.
I used 50 F to 100F because although you mentioned 40 degrees, the ambient air temps do not always equate to the water temperature, and I used 100F because most people will get in when it's actually around 95 or warmer. I used 158,000 BTUs as I was not able to find a Rheem model 8000 and the largest I was able to referece was an model 160 (158,000 BTUs). You can play with the numbers BUT - use your actual numbers in the equation.
NOTE:
These are "ideal" calculations and does not factor in the decreased BTU output of the heat pump during cooler air temps, heat loss from the spa shell to ground, or the dissapation of heat of an air blower or the aeration of therapy jets.
In SOME cases, you're better off keeping the pool heated, so that when you need to use the spa, there's a much shorter period for the spa water to reach your desired temperature. What is very important is that you use a solar blanket to help retain the heat you've added during the day.
Compare this to maintaining your pool at temperature (again, with a solar blanket):
Your heat pump would run about 10 hrs/day during the coolest winter months to maintain 83 degrees, but the heat up time to use your spa would be much shorter.
If you're not planning on using your pool during the winter, it doesn't make sense (in this case) to keep it heated, for the sake of the spa. Just plan on turning your system on at least 2- 3 hrs before you plan on using it.
Hope this helps,