Hi Ellie, I'm Ellie, too!
There really isn't an average weight loss per week because there are so many contestants, plus you can't predict how well somebody's body is going to respond each week. Sometimes they can drop double digit numbers in a week (which may actually be longer than 7 days by their time); other times they may do the same strenuous workouts and lose next to nothing. Season 7's batch (this season that just ended) dropped some huge numbers in the beginning but they also started out heavier than previous contestants.
The length of time has differed through the seasons, too. This past one was the longest season of all...19 weeks of show, including the finale...timewise, the players were there from mid Sept 08-mid Feb 09, went home for 30 days, came back for a weigh-in and a marathon and went home again until the finale earlier this month. But that's basically 5 months spent mostly at the ranch. Season 6 only lasted 13 episodes. I think their taping was roughly mid-May 08-mid-Sept 08...they had already gone home by the time the show even began airing, then they had the live finale in December, so those finalists had 3-4 months at home before the finale.