It has been estimated by DICE that it will take an approximate 100 hours to unlock everything in Battlefield 3.

Senior Gameplay designer Alan Kertz posted this on Twitter:

@Demize99
Alan Kertz
@450bq If you unlock every weapon, every gadget, and every unlock for every weapon… something like 100 hours.

DICE also explains why it would take so long to unlock everything:

The reason it takes so long is because unlocks in Battlefield 3 are given per weapon basis, where each weapon attachment must be unlocked for a specific weapon, meaning that players will be unlocking the same attachments and gadgets over and over again, for each weapon. Most weapons will be given three slots: one for scope, one for barrel, and one under-barrel slot for grips, bipods, grenade launchers, etc.

On one hand, the more you use a weapon, the more and better accessories you get for it, on the other hand, it might get a bit too repetitive unlocking the same thing again and again. But we’ll have to wait and see how the system works when the game ships. Or in the upcoming beta.

Good news for hardcore overachievers, bad news for regular gamers. Most of us will never be able to unlock everything if we play it casually.