You will need to be sure that roots are growing, too. If you find the leaves are growing without roots, that's not a very good sign and it is likely you will ultimately fail. If this is the case, close up the bag and wait another month, then check again.
Stick a fork in the soil from the side of the cutting, then under the cutting to loosen the media so you can gently lift it out and look. If it were me, I would just leave them for another 2 weeks and then check. when you deem it is time, never remove the bag outright. First open the bag so there is a 1 inch hole for a week. Then a 4 inch hole for a week, then open the top completely for a week. Then you can remove the bag. If the cuttings show any stress during this period of adjustment, go back one step and repeat.
