Can't argue that advertisements are an extra means of a web site generating income but should web designers take advertisements into consideration when laying out the pixels for a site? Should this be discussed in a design brief? How can you determine when advertisements are enough? Is their a ratio to consider when adding advertisements?
I think it would definitely depend on the brief, but you never really know if any site will eventually need to resort to ads, so it's better to be prepared.
What I usually do is plan (even if it's in my head!) where those ads could be. There are some common places - both literal and metaphorical :). Main container, top banner, left-side menu, right-side menu... you should consider you might need to add elements in these places and design accordingly.
Google's Ad Sense has some nice guidelines on ads placement. Some worth mentioning are choosing ad styles that are easy for your users to read, defining Ad Units and keeping the ads looking like ads. There are also some penalized practices to consider, like too heavy ads in the top.
I don't think there is a ratio, it really depends on the type of site. Some pages highly depend on ads, so you will need to design around them, rather than having them as add-ons. This article also seems to have some nice examples. Like Ad Sense mentions, sometimes less is more. Keep your ads looking like ads, and make sure there is a balance between ads and content, and you should be in the right path.