As with many insects, the time that it takes for leaf insect eggs to hatch depends in a large way on the temperature that they are kept at. Generally speaking the warmer the eggs are kept, the sooner they will hatch because the tiny baby insects inside them will have been able to develop quicker.
Kept warm such as on a reptile heat mat it is normal for leaf insect eggs to take between 3 and 5 months to hatch. 4 months seems like a good average though some people have found that their eggs take far longer to hatch so don’t get despondent if you have been waiting a long time. Never give up on those eggs 🙂