Electric Fan turning on in moderate traffic...how often is "normal"?

[CLS70]

2007 X5 3.0si

I recently changed the electric pump and thermostat after a failure (1 months ago). I also replaced several hoses and used new coolant.

So far the car has been working fine (knock on wood), however, I have noticed a few times in moderate stop-n-go traffic that the electric fan gets activated. e.g. after 10-15 minutes in traffic, I hear the fan activating, it lasts for a 3-4 minutes and then turns off.Outside temperature is not high (20 degrees celsius).

I have already checked coolant level, all hoses and everything seems ok (or at least could not see any leaks).

Maybe I'm just paranoid but would appreciate hearing from others how often and under which conditions the fan is activated or any suggestions on what else should I check...

Thanks,