What is the policy for advertising on userinfo pages?
There's no official policy, and this is dealt with on a case by case basis. Until the user attains a certain level of reputation, all the links on their page are NOFOLLOWED so the search engines don't give those links undue attention.
Beyond that, the system works pretty well.
- If they don't contribute, then no one visits their user page, so the ads have no effect.
- If they contribute spammy answers/questions, then the spam/abuse flags are set by other users on their posts, and the moderators generally take a look at the account to see if there is a pattern of abuse before determining the best action to take.
- If they contribute good material to SO and:
- Their ads are not offensive, then all is well and they are receiving a benefit from their work on SO
- Their ads are offensive, then whoever got offended is welcome to report the account to email@example.com, or flag one of their posts and note that their user page is offensive. It will be reviewed.
How much is too much?
There is a limited amount of space in the user page, and not all of the fields are free-form, so at most any excessive advertising can only take up perhaps 50% of the above the fold space, and 25% of the overall userpage.
If it's not offensive, then there's really no reason to complain - no one will come if they don't contribute to SO, and if they contribute a lot of good stuff then they will get a few hits on their user page, but it still won't be significant in the scheme of things.
If they find a way to subvert more of their user page into advertising, and somehow draw people in, then that's abuse of the website, and should be reported on a case by case basis.