She could be restless for some hours before a lamb starts to emerge, but usually between one and two hours from wet bum to seeing feet.
If she seems to be straining from time to time, that’s good. If she stops straining and goes back to grazing and cudding, that might not be good.
Once you can see feet, you just want there to be progress. If nothing progresses for more than half an hour, time to offer a gentle assist.
If you have reason to expect overlarge lambs, or more than two lambs, or any other factors which predispose to difficult lambing, you might start to worry sooner than you would if it was, say, a Shetland gone to a Shetland tup.
The biggest worry if you don’t see any sign of feet in the next half an hour or so (writing this at quarter to one, so over an hour since she had a wet back end), is that it’s stuck, breach, or a dead or sickly lamb. Stuck or head first might need a leg pulled forward, breach will need delivering quickly, a dead or sickly lamb often malpresents. And in all cases, any lamb behind could get into difficulties, so you don’t want to wait too long to assist if it’s one of those.
If no sign of feet, or if she seems to be giving up, in half an hour or so, I’d get her penned and have a wee exploratory feel. If the toes and nose are within reach, you could probably let her try a bit longer. But if there’s only one foot, or no feet just a head, or a tail, or back feet, then you’d need to deliver.
Hopefully by the time you read this she’s already produced and is licking her lamb(s)