Unless you plan to naturalize, you can stay for up to a year w/ a GC. If you intend to naturalize, you'll need to return back within 6 months to prevent your waiting period from getting reset. (For Eg: If you've been a Permanent Resident for 3 years, and you stayed in Nepal for 7 months, you'll start from Day 1 upon your return). You can get Reentry permit, also obtaining a returning resident visa which will allow you to stay for up to 2 years.

However, your GC will get terminated if you declared yourself a "nonresident" while filing your tax returns.