I reckon you'd be best to take as long as you can - I generally put them in the tank, in their bag to heat up, then go watch telly. Each advert break I remove some water from the bag with a glass, tip it down the drain, then refill the bag with the same amount of tank water. After about 40 minutes, I net the fish and put it in the tank - that way if their is anything nasty in the transport water most of it will get flushed away.