Why, where and when does Jeff leave?

He apparently decides to go to the island of Lesbos in Greece, because he thought (because of the name) that the island would be full of lesbians.
He leaves somewhere between season 3 and 4!

Jeff leaft in the 1st episode of season 4 - he wasn't in it, but Steve had a phone call from him

Too visit the island of lesbians (as he misunderstand the name lesbos). This is a attempt too see a naked woman again. But then again he leaves too soon.

In episode 1 of season 4 (9 and a half minutes), Steve talks to Jeff on the phone about critical mass in breast implants while on a flight to the Greek island of lesbos