They can vary enormously but if it's made no progress by tomorrow, I'd be tempted to help. They can find it hard to zip for whatever reason although the only times I've had to help chicks is when I've been faffing around with the incubator to get earlier chicks out and the humidity changes have stopped the later hatches. If you do decide to help, just start picking off the shell a tiny bit around the hole he's already made. Once you've picked off a bit, leave it a couple of hours before picking off some more. If there's blood when you pull off the membrane, leave it longer.