dd is 8mo, pretty rubbish sleeper but of late has been waking 'only' two or three times - usually at about 12, 2, 4 then up at about 5.30 or 6. I give her a feed at 12, but the other wakings I'm hoping she might not be hungry and ultimately we can get DH to settle her if she still wakes - we've realised separation anxiety isn't helping things atm.
When she does wake there's usually quite a lot of that low level grizzling, which I can't sleep through although Dh can. We've just moved house, neighbours also have kids so have been very understanding and also dd's room is on the outside wall so chances are they can't hear her anyway. But how long is reasonable to leave and hope she takes herself back to sleep? It often doesn't escalate too badly until one of us goes in to pat her and help her back to sleep at which point she goes ballistic and needs boob (sigh). We've let her grizzle mildly for up to half an hour before, by which point I basically say, she's not going back to sleep, we need to do something.
With dd1 she had sleepy music on her listener which was part of the bedtime routine and we would just put that back on, usually she'd quiet down again very fast. Bcause dd2 has only just moved out of our room she doesn't have that.
I'd say 10 minutes for actual crying, but for the grizzling, you know her best, whatever you are comfortable with. I let my DD2 grizzle and cry a lot more than I did for DD1 partly because I have to (while dealing with DD1) but also because I know she will often settle herself where DD1 never did.