@Aline it is just a matter of using a razor to gently make a slit in that outer shell, keep the humidity fairly constant. From old photos on our laptop I worked out they should take around a month before mum chews a hole in it, I'd imagine temperature and humidity will affect hatch times.
Spiderlings generally, will eventually get out on their own but if your worried you can put a small slit in the outside layer to help things along.
At least you could have a peek and see if they are infertile
I've found these take around 2.5 to 3 months to hatch, hatching often seems to occur after a good misting. I mist mine every couple of days or so and the slings will make a hole themselves to hatch out from.