How long should i wear my invisalign for as a retainer?

I recently got invisalign to act as a retainer, not braces, for my teeth. My orthodontist said all day every day for a year, then at night for the rest of my life. On the other hand, my friend, who has worse teeth than me, was told by a seperate orthodontist that she has to wear it for a week all day every day, then only at night. I trust my orthodontist however he hasnt always come up with the best ideas (something called a herbst appliance) or been straightforward with me. So what would be the average and also, what would be the bear minimum.