I would say that after 10 minutes is acceptable. I have put other peoples clothes into a laundry basket so it is all contained for them when they return. I have had this happen to me when I went and did something else and didn't hang around waiting for my clothes to finish washing or drying. It didn't bother me at all and I feel that procedure is acceptable. Otherwise, I should be there when my machine finished running.
On edit, 10 minutes is reasonable.