It is OK to stay less 180 day for single time.For 6 month to one year, you have to convince the immigration office on airport, you do not want abondon you residency in US and you still live in the US.For one to two years, you have to apply RP before you leave. For the safty, I think it is better to apply RP if you want to stay outside US for over 6 month and less two years
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