DS(3) is exactly the same. I have gone potty training cold turkey with him today after a false start after christmas.
So far today he has wet his pants at 10am and has done nothing since - he has now been sitting on the potty quite happily for an hour and a half with no results. No quite sure what the right thing to do is!
Don't make them sit there for ages. That is cruel. Get them to sit and try every ten minutes. If they don't go after a minute, they can get up and carry on doing what toddlers do. Also they should not be wearing a nappy in between - pants or nothing. In my opinion.
If you get nowhere after a week, give up and try again in a couple of months.