It depends on the person, the break-up, each time it’s different. My last break-up was after 2 and half years, and I was pretty much ready to go out with someone that night. I met someone a week later, and it turned into a serious flame, not a rebound guy or anything.
It just happens, one day you see a cute girl or guy, you talk to them, you grow enamored and poof, you’re ready to go out.