Community, and when you join the squadron in its workup-deployment-standdown cycle. Most of your hours come during workup periods on the road (Boat and Fallon) and on cruise. In between boat/Fallon periods on workups, and during post-cruise standdown, nobody's flying, especially you, New Guy.*
Best time to join, hours-wise, is right at the start of workups, or right before cruise. That'll probably give you a cruise and two full workups, or two full cruises and a workup cycle.
That's all assuming your Air Wing is on a good ol' 18-month cycle. Sure wasn't that way the last few years - I did two full workups and a cruise-and-a-half in 36 months, and hit 1000 total hours about 2 months before I left. I've got about 960 E-2 hours.
As always, YMMV.
*edit: except for FCLPs...E-2s usually get the unGodly-hour-of-the-night periods at Fentress for noise abatement reasons, which is always fun