You'll have to open the bag gradually, just a bit every day or apparently the cuttings will dry out. You'll have to experiment w/ it to find out (sorry, I've never grown 'em in the bag).

I grew most of my Fittonia in terrariums, or just in the more humid bathroom.

Best sign of their having rooted will be new top growth.

Don't know if you have the large or small leafed Fittonia, but from my experince, pinching them back often is recommended, like when they get their 3rd pair of leaves, I'd pinch the tip off, & stick it back in parent plant to root.

Ok. I will give opening the bag slowly a try. I'm not sure what size leaf fittonia I have, but the leaves of the cuttings that I have taken have grown considerably, but no new growth has appeared yet.

Well, good then, if the leaves are getting noticeably bigger then you're on your way. It may be a bit tricky to lean the right humidity so they don't dry out after the bag is gone.

Update: day 4 with opening the bag slowly. The bag is dry (no extra humidity) and it's pretty much completely open. I will probably leave it in there for another week or so before trying to remove it. I'm keeping my fingers crossed.