A majority of sites pay by sale (CPS) rather than click (CPC) these days. I've seen an average conversion rate at the seller end of 2% quoted somewhere (this will vary by industry, of course). On your end, conversion will be defined by click through. So actual sales per month might be calculated like this:
actual sales = total visits X affiliate conversion rate X seller conversion rate
Suppose you have 10,000 visits per month to your site, the affiliate conversion rate (clickthroughs) is 5%, and the seller conversion rate (actual sales) is 2%. That would mean 10,000 X .05 X .02 = 10 sales per month. This obviously won't be a way to get rich quick from your website, unless you have a lot of visitors and/or the return to you per unit sold is quite high.
Be aware too that popup and ad blockers are becoming very popular ... I use one.

This will affect the affiliate conversion rate for many ads. Personally, I'd be very selective about ads and choose to be an affiliate only if the ad provides a product or service of genuine use to your visitors, and thus enhances your site (like an Amazon book that fits in well with your site's theme). View any income from it as a nice bonus rather than an income stream and you're likely not to be disappointed.