According to wikiHow.com
As all pool owners know, it's a complete system that needs maintaining so that your cement pond stays crystal clear and refreshing. Water clarity is a combination of maintaining the chemical balance of your pool, and proper filtration. This article will help you get a handle on your filtration needs, and how long the filter should be run for optimal effect. Read on!
Step 1
Use this equation: (Pool Volume ÷ Filtration rate) x 2 = Hours to run filter. This will tell you how many hours to run your pump to filter the water a full 2 times. Let's break that down:
Step 2

Know the volume of your pool. How long you run the filter depends on the size pool to filter ratio.

If you have a small filter you will need to run the filter longer, depending on how many gallons the filter can process in an hour. (Google the flow rate of your pool pump.)

Calculate the volume of your pool by multiplying the length x width x average depth in feet.

Multiply this number by a standard multiplier which is 7.5 for rectangular and square pools and 5.9 for other shapes. Example 16*32*5*7.48= 19,149. This would give the volume of the pool in gallons for this 16x32 pool that has an average depth of 5 feet (1.5 m).
Step 3

Determine the flow rate of the pump. Include the resistance to flow in your plumbing system. (Google the flow rate of your pool pump.)

The pump manufacturer can tell you the flow rate for certain resistances.

You can estimate your pool plumbing resistance to be 20 ft/lbs for small pools, and 40ft/lbs for large pools or installations where the pool pump is far removed from the pool area.

Waterfalls and elevated solar systems will cause the ft/lbs resistance in the line to increase.

An average 1 HP pump will move about 50 gallons (189.3 L) per minute. This would be 3,000 gallons (11,356.2 L) per hour.
Step 4 Calculate the turnover rate for your pool:

The minimum recommended turnover for a pool is two complete turnovers within 24 hours.

For example, using a pool volume of 19,149 gallons (72,486.9 L), and a filtration rate of 3,000 gallons (11,356.2 L) per hour:
(Pool Volume ÷ Filtration rate) x 2 = Hours to run filter
(19,149 ÷ 3000) x 2 = 12.766, or about 12 hours and 45 minutes for two full cycles.