I know that a green card holder can stay (without problems) 6 months out of USA.
But after returning to USA, HOW LONG minimum should one stay and leave again without having problems?

There is no set time. Even the 6 months is not exact, there are many instances with problems at the POE for LPR's outside for 4 months. You can leave the next day if the circumstances warrant.

You need to take the time to document why you are spending so much time outside the US and that you still maintain strong ties to the US. You should be filing taxes every year, maintaining bank accounts, employment, real estate, club memberships, etc.