I believe his question is, how long should the snake be left alone in the tub before he puts the hopper in with the snake.
I would put the snake in first, then put the hopper in right away.
Once the hopper and the snake are both in the tub, immediately wash your hands. If the snake has not eaten in 30 minutes, remove the snake first, return it to its enclosure then remove the hopper and put it back in the freezer.
I say remove the snake first because you want to handle the food as little as possible to avoid forgetting to wash your hands.
I would only try to feed the same hopper twice before throwing it out.
I agree with snake Charmer on the last part. leave the snake alone for about 15 minutes after it eats and then put it right back in its enclosure.