50-100 clicks will usually give you enough data to begin analyzing.
Before you even start getting clicks, though, you should carefully consider your bidding - the max per click you are willing to pay. Let's say, just for example, that you are promoting a product where you earn $13/sale.
To start, factor in a 1% conversion rate. This is a typical rate, anything more than that would be great. So for every 100 click-through's, you make one sale and earn $13.
Obviously if you bid 13 cents per click, you will break even at 1%. So you know starting out that you must keep your bid below 13 cents. This is just a generic formula to determine bid ranges. Hope that helps!