It will take 'something like 100 hours' to completely unlock an entire kit in Battlefield 3 report bf3blog, . The number was confirmed by senior gameplay designer Alan Kertz on twitter this weekend. Responding to gamer 450bq's question "How long would it take on average to complete a kit?", Kertz replied "If you unlock every weapon, every gadget, and every unlock for every weapon... something like 100 hours."
DICE are clearly aiming for longevity in their unlock system, and we'll be able to see how well that philosophy holds up when Battlefield 3 is released on October 25th.