I've been part of both communities for half a year now and despite the fact that I've not being particularly active I am surely interested in the good practices in posting questions in here and the other exchange.

What sort of questions are supposed to be asked in MO compared to the ones asked in SE? Is it okay to post undergraduate/graduate level mathematics questions on MO and not on MSE which is research-oriented or is it the other way around?

Also, not aware until recently that cross-posting is not widely accepted (I've done it for 2 questions I posted in the past 4 months, again, unaware of the fact that is not good-practice) what are the cases for it to be allowed? How much time should one wait for its unanswered questions to be cross-posted on the other exchange?