If never before flushed OR it looks brown or dark or smells at all burnt, change it. Then keep your eye on the fluid for the next year or 2 unless you develop a specific symptom you need to act on.

I changed mine at 60000 and 120000. Each time it looked kind of brown, obviously not fresh but not burnt either. I have got occasional rough shifts time to time since buying the car at 30000, unrelated to changing the fluid. I have experienced this on other cars too (Volvo and others). If the trans is going bad it will tell you, though it might not give you much advance notice.