Online games in general have a large amount of unlockable and optional content in order to keep the players interested in it beyond the usual gameplay which can get repetitive fairly quickly. Battlefield 3 isn’t going to be an exception, with 100 hours of gameplay needed to unlock everything.
On his Twitter account, Alan Kertz has said to user 450bq “If you unlock every weapon, every gadget, and every unlock for every weapon… something like 100 hours.” in response to “how long would it take on average to complete a kit?”, a very sizable amount.
According to BF3blog, the reason it takes so long is because unlocks in Battlefield 3 are given according to the weapon. You don’t unlock a laser scope, you unlock a laser scope for that particular sniper rifle. This means you’ll be unlocking the same gadgets repeatedly as you find a better rifle for camping in that one spot.
This means that the more you use a specific weapon, the more your options are expanded on that exact weapon, this can result in a weapon being a bit of a Magikarp, where it starts off useless and once you put up with it long enough, becomes a killing machine that headshots from the other side of the map.
This adds a long amount of playtime to the game but I feel that having to unlock everything on a specific weapon and being unable to use those resources on another weapon is a bit of an immersion break, and also artificially pads content. Still, we can wait and see how well it works and how long it takes to fully unlock a single weapon. If there are 30 weapons, then that’s a very reasonable timeframe on average.