You calculate your dymanic head loss, look at the flow chart for your pump, follow the grid to where your head loss intersects pump flow, which tells you how many gallons per minute you are getting.

Then, you dividide the total gallons in your pool by the gallons per minute of your pump, then divide this answer by 60, as there are 60 minutes in an hour. This gives you the number of hours it takes your pump to provide 1 complete turnover of your pool water.

At low speed, the flow rate is 1/2 of the high speed gpm, so it takes twice as long to get one turnover. However, you use only about 1/3 the electricity at low speed, so you are getting the same work done over a longer time for less electrical cost.

As for figuring out your dynamic head loss, hopefully Mas will be along soon, or else you can read his "are you losing your head" sticky in the pool equipment forum.