"if someone received cg a few months ago, how long can he legally stay in the US if he were to discontinue the job to take a break ?"

You are asking a weird question. Once you get your GC, wether you have a job or not, you are legally a resident of America and can live in the USA independent of your job status. That is why it is called "Permanent Resident."

If you don’t have money even a citizen can’t stay without job for a day.You can stay under the bridge or at the side of the street though

With temporary residence permit ( TRP) you can stay upto 6 months without contributing SS . But with greed card ( PRP) or PR there is no restriction . U can be employed your whole life , if you can survive.