u/DefATriangle

So I have read Eragon about a dozen times by now and I have the same question every time. It took a few weeks for Saphira to hatch from the first time Eragon touched the egg in the spine. How did the riders know when a dragon in the egg chose the right person. Was Saphira a special case or did every candidate hold an egg for a few weeks? I need to know the answer.