Today's total hip replacements are indeed done in an average time of 75 to 90 minutes. The typical range is anywhere from 45 minutes to 120 minutes. The time factor depends on the type of problem (fracture versus arthritis) and type of implant.
Some patients may only have a partial hip replacement called a hemiarthroplasty. Others have a total hip replacement (THR). The method or technique used can make a difference. In a minimally invasive (MI) procedure, the surgeon makes a much smaller incision compared to an open incision. MI hip replacement operations can actually take longer than a THR with an open incision.
Hip dislocation is a potential complication after THR. It doesn't happen very often. When it does occur so early after the operation, it's likely due to a fall or poor positioning.
Patients are given strict instructions on motions and positions to avoid. Sometimes they forget and cross their legs or bend the hip too far. Dislocation can also occur when turning in bed if the patient isn't careful.
It's very unlikely that there is any link between the operative time and post-operative dislocation. You should talk with the surgeon and get his or her impression of the cause of the dislocation. The cause may be unknown or there may be a simple explanation.