According to the Wizards Optimisation forums for 4e:
"What I've observed is: 14 DPR(1st), 20 DPR(6th), 40 DPR(12th), 60 DPR(16th), 100 DPR (24th), 150 DPR(30th)"
DPR meaning damage per round.
If you're going to go "that's not possible", one of many character builds that accomplish it is here, and it's not even a particularly munched out or complex one to do. There are also builds that do more damage.
Even at first level, though, you can expect to take out a guy every two rounds with a decently-made striker, and damage increases ten-fold from there to level 30.
Note that you can't divide HP/DPR = average rounds to take out a monster. The average rounds to kill a target is >= HP/DPR.
The reason why is that you typically kill a monster "with overkill." Only when there's never any overkill does HP/DPR= average rounds to kill the monster.
The difference can be significant: see calculations here
For example, an attacker with a 55% chance to hit (including a 5% chance to crit), dealing 1d12+13 on a hit, plus an extra 1d12 on a critical, averages 5.22 rounds to kill a defender that is equally likely to start with 42-55 HP.
In this case, DPR= 11.325, so HP/DPR= 4.28. The actual average rounds to kill the defender is 22% larger than this formula would indicate.