I'm interested in getting some opinions about how long the damage to the bathroom ceiling in the photo would have taken to accumulate. The source is a small drip in the ceiling that comes and goes. Note the warping in places.

 

In my non-professional opinion it would be difficult to say whether it took months or years but I would venture that it couldn't have happened in days or weeks. Your thoughts?

First question have you stopped the area above the ceiling that has the drip? If not you should. Secondly a few months depending on what a "small drip" actually looks like.