I don't think there is one. There needs to be a limit in order to prevent MySQL coming to a halt and all Web sites suffer from it while it does it's queries.
In a way it sort of makes a host provider limit how many busy sites are on the server (though not a good indicator as some providers just don't care how slow your site is regardless of what is causing it)
If you do see this limit pop up then you probably do need to change providers or at least think of a more dedicated solution.
