I'd say around 6-7 hours. There are alot of people that are slow/bad at dging, some will definitely end up in your party unless you do some sort of clan event.
But it depends who you leech from. Leeching from a trusted clan can do floors under 20 minutes, or even 15 minutes. That's around 200k exp per hour. But then again, I'm not sure if they open all doors, so exp is more like 180k per hour.
Some people lets you leech for a slightly lower price (2M) and they either don't have a guaranteed time, or they take anywhere from 25-35 minutes. That's around 120k exp per hour.