I thought it was generally accepted that it takes 3 years to build a squad. Pretty sure I heard Martin Johnson say it.
Also John Kingston, but I don't think anyone really listened to him...
But Ben Darwin's point is subtly different - his analysis of players moving teams suggests that it takes 18 months or more for them to fully integrate into a side and start performing at the level they were at before they left, and longer for props in particular because of the technical aspects of the setpiece (everyone emphasises slightly different things, and you have to learn to work with your fellow front rows).
It's hard for us to appreciate this when we're playing or coaching at an amateur level and individual talent can often overcome team cohesion, but at a professional level the margins are even finer and cohesion counts for so much more.
To give two examples - player for player there weren't many in the 2011-12 Quins team who were clearly better than the best of their opponents. NEv. Maybe Robshaw. But we did have a lot of cohesion and a team that started winning and learning to play off each other effectively and it was enough.
More controversial, perhaps, but the All Blacks aren't that much better than their opponents individually. It's their ability to keep playing their system right to the end of games that usually breaks down opponents in the end, even when they go behind. But on Saturday they came up against a team who could match them step for step in sticking to the game.