Life Expectancy: Ideal number of clicks for DSLR camera?

I'm planning to buy a used nikon d90 with about 15, 000 clicks.
what would be a reasonable number of clicks for a camera to be considered a "good" condition?
what is the life expectancy of a DSLR camera in general?

Most manufacturers say that the shutter should be good for 100, 000 activations or clicks. Most users do not come close to that number. As long as the cameras has not been mistreated or shows signs of physical damage, it should work fine. I would actually pay closer attention for any usual noises, battery corrosion, or problems with the buttons. Take a few shots with the camera and see if it over/under exposes pictures and autofocuses properly. Treat it if you were buying a used car.

There are professionals who claim a million "clicks" or more. 15, 000 is a drop in the bucket.

But I would caution you about used gear. How has it been treated? How have the lenses been treated?
Beaten or roughed up? Wet? Have they been stored in an appropriate bag all the time when not in use? Lots of questions with used gear.

If you know this person, if you can hold, examine and shoot with the camera prior to buying then I'd say as long as you are satisfied go ahead. But be careful.

Good condition goes beyond shutter clicks. I think the d90 was rated on 100000 clicks.

100.000 will be a good reference number.