After 180 days you're deemed to be seeking a new admission under INA 101(a)(13)(C) which means you're subject to the grounds of inadmissibility on that basis alone. [1] And after 1 year away, it's not a "you should get a REP" it's a "you definitely have to unless you have a medical reason for not being able to come back sooner because they won't let you in without a returning resident visa."

> A permanent resident (called lawful permanent resident or LPR) or conditional resident (CR) who has remained outside the United States for longer than one year, or beyond the validity period of a Re-entry Permit, will require a new immigrant visa to enter the United States and resume permanent residence. A provision exists under U.S. visa law for the issuance of a returning resident special immigrant visa to an LPR who remained outside the United States due to circumstances beyond his/her control. [2]

More than 6 months interrupts your continuous residency for citizenship purposes unless you have a REP.

Personally I'd err on the side of caution and say if you're regularly away for long periods or if you plan to be away for more than 6 months, it's probably worth the REP. I think any extended or regular absences should probably be regarded with a little more weight than 'it's fine' although most people probably do. If I were gone for more than 6 months I'd expect a pretty significant grilling.

[1] https://www.ilrc.org/sites/default/files/sample-pdf/inadmiss...

[2] https://travel.state.gov/content/travel/en/us-visas/immigrat...