I try and wait about five seconds after heavy is up for teammates. If no one is on the way, I grab it and go. Why? I figure it takes 7-10 seconds for one person from the other team to reach our heavy ammo, heavy weapon in tow. Some matches happen where my team is busy assaulting the enemy heavy ammo crate (and loses 9 out of 10 times) and by the time they all day and respawn, I collect the crate. Then the hate ensues. BUT, every time ONE person is showing an indication that they want in on our crate, I wait.