As much as possible especially if sex is involved.
There is no "should". They should meet as often as they feel like.
This varies hugely depending on the people. If either of you are not happy with the set up then best have a conversation about it. Best relationships have great communication imo
As often as they both want.
It would differ from couple to couple. Some see each other every day, some need breaks Talk about it with your other, and find out what suits the two of you.