It depends on the niche Example: Dell advertisements from computer niche: Say the CTR is 1%, 100 visitors = 1 click. 1 click = $1 Clothes: CTR 1%, 1000 visitors = 10 clicks. 1 click = $.10. 10 clicks = $1
In my opinion, you need at least 2K uniques. If you own multiple sites, this is much easier to attain.