When it’s possible, the best time to meet somebody is when you don’t need to meet them. In The First 90 Days, Michael Watkins writes:

Remember: you don’t want to be meeting your neighbors for the first time in the middle of the night when your house is burning down.

There’s also a big difference between somebody needing to follow up with you—they were probably hoping they wouldn’t have to—and you getting ahead of that by being proactive. The best time to communicate is the time you don’t need to. 

Conversely, you don’t want to correspond only when you absolutely have to. (Don’t let that stop you from asking for help when you need it though!)