How long can green card holders stay outside the U.S.?

You are supposed to reside in the U.S. as a permanent resident. If, for any reason, you need to stay outside the U.S. for a sustained period of time, say, more than one year, then consider filing necessary applications for such a period. Consult an immigration attorney for further guidance on your plans.

I would limit it to less than 180 days at a time, or get a reentry permit first.

The intent of conditional permanent residents or unconditional residents is to consider the U.S. as their permanent home. Being away from the U.S. for more than six months during a set period of time may create a presumption by the U.S. CBP that the returning resident may have abandoned their permanent residency.

Yes. If you plan to be out for more than six months or for reasons that indicate the U.S. is not your home, you should apply for a reentry permit.

Yes, they are. A reentry permit allows you to remain outside for up to two years during the first few years. Then you can renew it, but you must be in the U.S. to file the application and provide fingerprints.

Yes, conditional permanent residents are also subject to the same physical presence/residence requirements. If you remain abroad for more than one year, it is considered an abandonment of lawful permanent residence. In order to avoid that, you can file for a travel permit declaring your permanent resident intent in the U.S. while traveling extensively abroad.

Normally not more than six continuous months, unless you first apply for a U.S. re-entry permit prior to your departure from the U.S.

Yes. In general, you should not stay outside of the U.S. for more than six months unless you have the reentry permit.

They have the same rule as unconditional permanent residents "10-year green card holders".