Seriously, hrmm it truely depends on your body type and a bunch of different factors. There is not an acutal number of days or miles, but its more of a process of trial error. You might spend money on 10 saddles and the 11th will be the best. Or if you are lucky the first saddle you buy works perfectly. And really listening to other poeple say that this saddle is really nice and comfortable, doesnt really work. From your riding so far and your experience, you can cut out alot of choices.

For example, I had this saddle that came with my bike. I absolutely hated it, it had a cut our for my nuts. It felt like i was killing them. But i made sure it wasnt just breaking the saddle in. I went over 300 miles with and could not stand any more. Then i realized i like really flat saddles. So i made my choice with a brooks swift. I absolutely love it, expensive but i never experience any pain.

I would say, evaulate what you like then choose. But with every choice make sure you give it a chance;' dont be quick to turn down. I guess thats why you see alot of saddles on ebay, peopepl are gogin thur the same process you are.

Good Luck.