Outside, check first light for new lambs and anyone thinking about lambing soon. Then broadly every 2-3 hours, more often of course if anyone is clearly preparing to lamb, or if the weather is such that you need to be checking newborns within 20-30 minutes of birth.
Last check before dusk, giving yourself time to bring in anyone who looks like lambing into bad weather or likely to have a problem.
I never like leaving newborns I haven't seen feed, but sometimes you have to use your judgement whether bringing them in creates more risk than leaving them to get on with it.
And yes, the sheep prefer that you don't go out disturbing them with a torch.
By far the majority of lambs are born shortly before or after dawn, and next most popular is around dusk. But enough do it in the middle of the day to mean you can't risk not checking between 10 and 3! lol
Indoors, every two hours at least. Overnight you can usually get away with longer - but be prepared to get caught out occasionally! You have to find the right balance for you and your flock. If you are so diligent you become exhausted, that's not good! If your indoor space is very roomy and your sheep generally don't have problems, you can leave it longer. If the sheep are fairly packed in, or you sponged so expect a lot of lambs in a short space of time, you really don't want to be leaving it long enough for two or more to be on lambing and getting the lambs jumbled up. So in the latter case, I'd do checks every 2-3 hours through the night too.
Of course if you only lamb a very small number of ewes (as we do this year - just 7), and didn't sponge or tease, then the likelihood of several lambing at the same time is pretty small, so you can leave it a bit longer. But if they are first-time mums lambing indoors, you don't want the lambs wandering off trying to suckle the wrong ewes...
My indoor lambing experience was in Northumberland with with Mules (so fabulous mums but a high incidence of triplets) and any Swaledales we thought it better to have indoors. Different sheep, different risk factors, so someone else might find 4 hourly checks are fine for their sheep and their system.