How often does the average person paint the interior of their home or condo?

It depends upon the demographics.

I know people with very large homes who have their decorators choose a new color scheme every year or two, whereas other people in more modest areas may paint every 10 years or more.

It really depends upon what you are calling an "average" person.

Only time we painted was when first moving into a place to suit our tastes and not the last person living there.

Average being "more modest" homeowner.

When it looks raggety

i say when needed