It could be 30 minutes, it could be 24 hours!
Try to be patient, and don't upset her by coming in to check
too often...
Once the water bag is seen (which sometimes you miss as it may burst very quickly, or she may walk around with it hanging off the back for hours like a gory fashion accessory
), it would normally be within the next 4 hours that she goes into labour, or could be very very soon.
Did you get your ewes scanned - do you know how many she has in there?