Maintaining US permanent residency

I'm USC and wife is UKC with US permanent residency. We are expecting to move back to UK in September once my spousal visa comes thru.
How long will her US residency remain valid? I know for the UK, it's 2 years before it becomes invalid, but wondered about the 'flip-side'.
Can she maintain her US residency by frequent trips back to US?
Thank you


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It's one year in the US.

No, you can only be a resident of one country.  If she applies for US citizenship before you leave, then you won't have to go through the immigration/visa process again in the future.