It depends on the Uromastyx. That is, each Uro is an individual and each will have it's own time frame for feeling secure with you. It also will depend on the species of Uromastyx you have. I have a pair of adult Sahrans I've had for several years, which will not take food from my hand. In fact if my hand moves anywhere near them when I'm doing maintenance on their enclosure they dart for cover. On he other hand I recently purchased a pair of Uromastyx philbyi and both the male and female were eating from my hand within a couple of days.

The two main things in getting a Uro to trust you is first, always let the Uro see you when you put food in it's cage. This way it associates you with good things. The second is patience. It may take a few weeks to a few months before they feel comfortable enough to come near you. Good luck!