For a multiple entry tourist visa, the admitting immigration officer has complete discretion over how long you may stay. There are some guidelines (or so I'm told), but if anything suggests that you are taking up residence in the US, you can be denied entry or given only a limited stay on any given entry while the visa is valid.

As has been mentioned, the best defense is to show your attachment to your permanent country of residence - business interests, appointments (medical, for instance) for which you must return, family, place of residence, etc. etc.
Cheers,
Bev