Let's tackle the why question first: The most common reason for draining a spa is because the amount of total dissolved solids in the water has reached its maximum allowable level. As a general guideline, spas should be drained when the TDS level reaches 2,000 to 3,000 parts per million or 1,500 ppm above the start-up TDS level.
By definition, TDS is absolutely everything dissolved in the water - from metals to chlorine to bromine, alkalinity, salts, bather waste and more. And when the amount of these substances in the water gets too high, it can cause a number of problems, including salty-tasting water, colored but clear water, algae growth, metal corrosion, cloudy water, eye and skin irritation, and false test readings.

What causes TDS levels to increase? When water is heated, aerated and agitated - as is the case with water in spas - the water's evaporation rate increases significantly. This, in turn, dramatically affects the TDS levels in spa water because only the water evaporates; anything else that may have been dissolved in the water gets left behind.
As the water level in a spa drops, the first prescribed action is to add make-up water. But while this water will have a relatively low TDS level, it too will evaporate eventually, adding its share of TDS to the remaining water in the spa. As this cycle continues, the spa water's TDS levels will continue to climb.
TDS levels are also increased whenever the spa water is treated chemically. For example, adding as little as 1 pound of dichlor, trichlor or bromine tablets to 500 gallons of water can create an increase of 240 ppm to the water's TDS level.
And it does not take long for that much dichlor to be added to the spa: In addition to the 1 to 2 ounces of dichlor needed for regular chlorination each week, many owners use their spas two or three times a week, each time adding another ounce of dichlor. At this rate, it would only take three weeks or so to raise the TDS levels by 240 ppm.
Many other common spa products also contribute to a spa's TDS count, including algaecides, algae preventers, acid, sodium bicarbonate, clarifiers, scale inhibitors, foam suppressants, stain inhibitors and nonchlorine shocks.
For example, if a spa owner adds a 2 ounce dose of a nonchlorine shock to the water after each use, that dose alone will produce about 28 ppm of TDS in a 500-gallon spa.
Another contributing factor here is bather waste. just 3 pints of human sweat can produce approximately 4 grams of salt, which would add approximately 2 ppm to the TDS of a 500-gallon spa. In the case of a commercial spa that may handle an average of 25 bathers a day, the amount of TDS from human sweat alone could be as much as 50 ppm every day.
And don't forget, bathers also bring with them residual soap, deodorant, hair spray, suntan lotion, body oil, hair gel, laundry detergent, hand lotion, cosmetics and other personal-care substances - all of which can contribute to the build up of TDS.
When you consider the possible factors that could contribute to the spa water's TDS, it's easy to see how the TDS level can climb very quickly and why frequent draining is needed.
Now, as to how often a spa should be drained, the National Spa & Pool Institute suggests the following formula: One-third of the spa's gallonage divided by the average daily bather load equals the number of days that the water can be used.

(Gallons x .33) .-- bather load = days of use

For example, a 1,200-gallon commercial spa with an average bather load of 25 people a day should be drained after 16 days of use:

(1,200 gallons x.33) -' 25 people = 16 days of use

For residential spas, however, most experts would agree that draining a residential spa every three months is appropriate.