1) Having a multiple entry visa allows you repeat visits as long as the visa is valid so yes you can (technically) visit more than once.
2) If granted entry, you will be given an I-94 form with a date on it by which you must leave the US. That determines your duration of stay, more on that here.
3) A visa only gives you the right to request entry at a country's border, the decision remains with border officials. It will be up to them to decide whether or not you're traveling too frequently. In the past you could keep the I-94 in your passport if you intended to travel again before it expired, I'm not sure that's still applicable but I couldn't find anything about a minimum duration between stays
Here's the Department of State on multiple-entry visas:
A visa must be valid at the time a traveler seeks admission to the United States, but the expiration date of the visa (validity period/length of time the visa can be used) has no relation to the length of time a temporary visitor may be authorized by the Department of Homeland Security to remain in the United States. Persons holding visas valid for multiple entries may make repeated trips to the United States, for travel for the same purpose, as long as the visa has not expired, and the traveler has done nothing to become ineligible to enter the United States, at port-of-entry.