Pipping - how long is ok?

As you will have gathered I'm hatching for the first time  .  I woke up this morning (7am) to find one of our eggs had pipped (day 20) shell missing but no big whole in membrane just a slit, where bits of feather and the occasional beak can pop in and out.  Well he's been active on and off all day, chirping away inside but not made any progress on the shell is this ok?  Humidity is set around 55% (after all of googling 55% seemed the safer opition, figures ranged from don't bother adding water to 86% most confusing), but I'm worried the membrane will dry out its white, with a very slight tea staining colour.  He's still chirping and you can see the membrane 'breathing'. 
Is it too early to worry , three more have cracked their shells (no actual whole yet) in the last two hours.
I'm going to go have a bath, while waiting for replies, better than trying to sit on my hands
thank you

thank you off to read.  Another had pipped whilst i was in the bath!

I only use a broody so can't advise you on that but from what I have read on other posts it does sound high.  Hopefully another member who uses an incubator will give you more advice.

It is safer at this stage not to open the incubator as you will endanger sll the others. Helping chicks out of shells is usually doomed to failure anyway. It is hard but you do need to sit on your hands. Good luck it will be worth it