I don't agree with the statement about the lamb being born within 15 minutes of it first appearing at the vulva. It is that quick in some breeds (eg Shetlands with Shetland lambs) but can take considerably longer in more meaty animals.
My rule of thumb is that I want to see progress every 10-15 minutes, so once the toes have appeared I would want to check if there isn't more to see each time I look, or if she seems to have stopped trying for a continuous period of 15-20 minutes.
And you need to check that it's toes you can see, that they seem to be front legs (soles down), and that the nose appears shortly after the toes appear. If there are no hooves, or if it's just one foot, or it's soles uppermost, or no nose appears following the toes, then you need to check her.
If it's back legs (soles uppermost), get on and deliver immediately in case the lamb takes its first breath inside the birth canal.
If there is no nose following the toes, then the head may be bent back and not in the birth canal, so get lubed up, find the head and bring the nose down on top of the legs.
No hooves usually means either full breech - tail first, all legs pointing back into mum - which also needs urgent intervention as she cannot get the lamb out unassisted with this presentation, or it could be nose coming, legs back, and you need to get in and push the head back in and bring the legs forward quickly, before the head swells and it all gets much more difficult to do.
It is also fine if the sac doesn't rupture until the lamb is born, as long as the ewe (or you if she doesn't do it) breaks it as soon as the lamb hits the ground. They don't need to breathe until the cord breaks, and the cord usually doesn't break until the ewe stands up / turns around. When I assist a lambing, I don't pull the lamb away from her back end until I've cleared its nostrils, and if it's been a hard lambing I give the lamb and the ewe a moment to rest before pulling it to her nose (which usually breaks the cord.) If she's a first-timer, I rub birth fluids over her nose so she has to lick her nose to clear them, and that usually triggers the instinct to lick the lamb clean. While she is doing that, I check that both teats are working.