Wouldn't it be nice if users always took the right path, did the right action when prompted, and answered your questions when asked? Unfortunately, the reality is very different. However, with the right timing, you can accomplish anything.
The best moment to pull the trigger is when the users just finished achieving its goal, and they feel like they just earned a new superpower from your product. That's the moment. But be careful, if you don't ask for the right thing, they might churn quickly. To be used with parsimony!