Pentre Ifan offer no problems for 24 hour access. As Kammer says, there's usually someone there - it's in the tourist guides and everyone seems to go there, even though they don't know what they're going for and are often confused/disappointed when they see it.

Think of it as the Welsh Poulnabrone!

The last time I was there I managed a 10 minute period with no one else - I was there for an hour or so. That was mid-summer during the day, though.