Here in the US, according to the FAA, a seatbelt must be worn during taxi, takeoff, and landing. There is no FAA rule saying to what altitude you must have your seatbelt on.
At MHSC in Longmont, CO, we have a rule of keeping seatbelts on until 1,000AGL, no exceptions! In addition to that, jumpers may not move from their positions in the aircraft until 2,000AGL.
In all reality, after takeoff, it needs to be decided by a competent S+TA, DZO, or DZM.
J.