Pins and needles while meditating is a common difficulty. When the legs or feet go to sleep, many students worry that they will suffer permanent harm. I've never seen this happen. But if your pins-and-needles feeling occurs in only 15 minutes, try not to be overly attached to Padmasana. Try a less rigorous posture such as Half Lotus. I also recommend the Burmese posture, in which the thighs are spread out so the knees can rest on the floor, with the feet tucked one inside the other. Many people find this easier than other cross-legged postures. Work on being able to sit with either foot inside the other.