I cannot speak for the system as it is now. When I was there in the mid-80's, it would take about a week for a letter to go from Brasil to the US, and about 3 weeks for a letter to go from the US to Brasil. Sometimes the mail arrives out of order, as in you get a letter that someone sent 2 weeks ago before you get one that was sent 3 weeks ago. When your missionary first arrives in Brasil, it will probably take several weeks to get the letters going regularly. Likewise, whenever there is a transfer it takes a while for the letters to get sorted out.
Packages can take a while.
One of my packages was intended for Christmas but it arrived in March, just in time for me to carry it back home. But I don't want to discourage you from sending a package. The church has asked missionaries not to use e-mail, except under special circumstances. I used to think that e-mail might be worthwhile in that messages could be sent so quickly. But, they might also become too much of a distraction.