Well, your guess is as good as mine. God forbid that it should actually say so on the visa itself!
I was so relieved to get the bloody thing a few months back that I didn't bother to look too closely at the grey slip of paper glued into my passport. I just assumed that it was what I'd asked for - the only thing I could get, according to the staff at the visa issuing agency in London: which is to say, a 6-month, double-entry visa, with a maximum of 90 days per stay.
Now that I finally get around to inspecting it more closely (because I am nearing the end of my first 90-day spell in China, and am planning to go to Hong Kong shortly to 'reactivate' it), I find that it doesn't clearly specify the number of entries allowed or the overall period of validity.
In the 'Entries' box, it merely says 'M' rather than giving a number. Presumably that stands for 'Multiple'; but there's no ready means of finding that out. And, with the Chinese authorities, there can never be any absolute confidence that 'multiple' even means more than one. The Chinese character in brackets after the 'M' (多) does appear to mean 'many'. But I'm not going to get my hopes up too much about that: there could well be unpublished 'interpretation rules' somewhere that insist that 'Multiple' on this kind of visa means (at best!) only two.
There's no final date of validity specified either. The only date given is an 'Enter Before' deadline for first use of the visa.
Now.... if that 'Enter Before' date can apply not just to my first entry into China on this visa, but to any subsequent entries; and if there is no further (unstated) limitation on the validity of the visa that might restrict the stated 90-day limit per entry; and if I really am allowed an unlimited number of entries on this visa (well, more than two; I'm unlikely to want to try to enter more than three or four times in total).... well, then, my visa might be valid until 90 days after that 'Enter Before' date - which is nearly three months longer than I'd expected, and would probably enable me to dispense with having to worry about applying for a Chinese visa ever again (since I am hoping to quit the place for good next spring).
In the past, I've always had one-year visas (usually with multiple entries, with no restriction on length of individual stay), so I haven't had to worry about such details. But now, they've suddenly become rather important.
But how the heck am I supposed to find that out? The visa agency in London has failed to reply to my e-mail requests for clarification of the visa period. And there's not likely to be anyone I can turn to here in China for a helpful answer on this. [Hm - the visa agency's FAQ on this begins by implying that the 'Enter Before' date is not a terminal date of validity ("You can enter even at midnight on the last day!"), but then immediately seems to say the opposite, though with a distinctively Chinglish lack of clarity: "Duration of stay is no longer useful once the period of validity expires." If I had my really snarky head on, I might argue that one's 'duration of stay' in China is never useful.]
And let's not even get into the question of whether that 'Enter Before' date is the last date on which I can enter or the day after the last date on which I can enter.
I am pretty sure it in fact means the day on which I must leave. But I'm going to make sure I leave on the day before that, just to try to be on the safe side.