Specific number of days you *need* to be in China for the visa?

Let's say you have your X, everything is registered (ie. Got the Residence permit, received your passport with the actual residence permit stating that your stay is until a certain amount of period) and you actually don't stay in China during the period of the visa- 1) are there any requirements on the number of days you need to be in China? 2) is the residence permit valid until the expiry date *even* if you are not in China? ie. Let's say you actually go back home to begin work early due to final classes end but you actually have to wait a few more months until official graduation.