You can leave as soon as your green card is approved. What you need to be concerned about is how long you stay outside the US before you return. If you stay outside for more than 180 days in a stretch, you are considered to be "seeking admission" which invites more scrutiny and potential reasons to refuse entry.
If you stay outside for 1 year, your green card will most likely be cancelled if you don't already have a reentry permit. And if you take a series of multiple long trips (for example 5 trips of 4 months each) with very short stays in the US in between, you could also have your green card cancelled even though each individual trip is under 6 months (in those cases you usually get a warning first).
