The Brazilian Tourist visa must be used (you must enter the country) within 90 days of issuance. Once you enter Brazil you can make multiple entries and exits and is valid for one year after your first arrival in Brazil. You cannot spend more than 90 total days in Brazil during this one year period.
The yellow fever requirement is not in effect at this time but may be reinstated.
My visa is still valid until February of '11. It was extended while we were in Brazil for a period of four years with no more than 180 total days in Brazil. To have it extended we stopped at the Federal Building in Sao Paulo. We were nearby sightseeing and my SIL said we could go there and get it extended for a number of years. U.S. employees from her company did it quite often. I figured it was only for Business visas but I was wrong. We went in and my wife explained that we would be visiting my sister a number of times in the next few years. The clerk gave us a document to sign, asked for our passports, opened them up and stamped them. The whole thing took about five minutes. There was about a $10 charge. It sure helped having a wife and SIL who spoke the language and knew the system.
The hardest part was getting the Brazilian visa but once we were in the country it was easy.
Take care,
Mike