About 30 minutes ago, I decided to try out frag grenades rather than concussive shot in multiplayer. (About 29 minutes ago was when I started kicking myself for not trying out grenades sooner).
I noticed that you can refill your thermal clips pretty much anytime - the ammo crates restock themselves faster than an Ubercharged dispenser. Grenades, on the other hand, aren't always there for the taking; usually, I have to go kill some dudes first.
What determines when grenade pickups respawn? Time, kills, waves, or something else entirely?