Crystallization is a natural characteristic of honey. The rate at which it crystallizes depends on several variable factors. Temperature is just one of them. The nectar source has a lot to do with it. The amount of pollen, dust, minerals, sugar and water content... just as honey color and flavors vary year to year and season to season, so will crystallization.
Warm it for as long as it needs to liquefy at 100*F to 105*F. don't rush the warm up for best, longest lasting results.
Fast, hot heating, anything over 110F can cook out enzymes and other nutrients, and ruin the flavor and aroma of the honey.