simple answer is there is no restriction as to when you can leave. you can leave anytime. its a free country and you are not an indentured servent and don't have any legal obligation to stay.

does this departure impact your green card process? that should be of concern.

this depends on where you are in your process. if i-140 is filed and approved then you can leave provided that your i-485 has been filed and its been 180 days since the filing. [ but you would need a sponsor to continue your case]. [ legally speaking, your new sponsor need to file a continuation of sponsorship with USCIS]

if you already got the green card then obviously.. there is no impact in the process right? process been adjudicated. so it does not matter.