Losing their gills and morphing can be very variable. It depends among other things, on their age, food availability, water temperature and stocking density. Light stocking with good food availability and low temps will usually prolong the larval stage, whereas heavy stocking, with less food and high temps will often make them morph at a very small/young age.
It's a good sign that they are taking a while to change, the bigger they are when they do, the easier to care for and feed they'll be as terrestrial juveniles