Generally it takes my group 6 people who all have been playing table top RPG's for most of our lives about 1 year to complete one Adventure Path.
That is assuming that we do complete the Adventure Path. So far were about to complete the Hell's Vengeance Adventure Path (I am running) we have been playing for about 10 months. We get together once a week for about 5 hours (would be longer, but were all adults with responsibilities)generally we chit chat for about an hour then start playing.
The problem I have with my players is not the time it takes to complete an Adventure Path. Its keeping them interested, a few of my players get board of there characters so easily and next thing they want a new campaign.
To fix that problem we alternate Adventure Paths, Example I will finish book 6 of 6 of the Hell's Vengeance. Then another player will start the Jade Regent 1 of 6 book when that is finished, we will start another Adventure Path and alternate between the two.