This is another question that is hard to answer. It all depends on how many times the spa is used per week, the amount people using the spa, and the care in balancing the water chemistry. There is a formula that can help you estimate on when to change your spa's water. According to an article from one of our industry top magazines, they have simplified a formula that is used for commercial pools and spas.
Here is the formula: take the spa's gallons and divide by 1/3. Use that answer and divide by the number of people using the spa everyday.
Example: the spa's gallons are 500. In addition, there are 2 people using the spa everyday.
  500 divided by 1/3 = 166.66 gallons
  166.66 divided by 2 = 83.33 days
  83.33 divided by 30 = 2.77 months
As you can see this 500-gallon spa being used everyday by 2 people will approximately last them almost 3 months. If you use your spa maybe every other day, your water may last longer. However, remember this formula is only an estimate and is used only to give you an idea. You may have special circumstances or unforeseen situations that may require you to drain your spa's water sooner. You must also remember that you will need to have the water chemistry well balanced for the longest lasting spa water possible.

HELPFUL HINTS: TO REDUCE THE AMOUNT OF SOAPS OR CONTAMINANTS TO THE SPA'S WATER, TRY TO USE THE SAME BATHING SUITS EVERY TIME YOU USE YOUR SPA, TRY TO SHOWER BEFORE USING YOUR SPAS, AND WASH YOUR BATHING SUITS IN THE RINSE CYCLE.