[Rainer]

There is no simple answer, but I think there is a common element to the "bad" kind of flipping, and that's dishonesty. If you tell a seller that you'll keep it forever but have every intention to flip it ASAP, its dishonest and should be looked down upon. If in that same scenario you really did plan to keep it but circumstances forced you to sell, then its not the "bad" flipping. So I think its less about how long you have it, but more about your intentions from the start.