The amount of mailings you send depends on the type of family your dealing with and how many responses you receive before your deadline(s)(hotel, transportation, etc.).
Since I come from a family that seems to get some sort of kick out of responding at the last minute (LOL), we send out a monthly newsletter and a postcard reminder, during the last 9 months leading up to the reunion. This way no one can say that they didn't know or they received the news too late. Of course, this can become expensive when sending via snail mail(postal). Yes, I still have a few family members who still rely on postal service(LOL). The majority of my family uses the internet/email to receive reunion news, and this helps to keep down the cost. During the last few weeks leading up to the reunion we send as many messages and reminders as we feel necessary. The internet can be a great tool to market the reunion to family members and keep them connected! We usually include a schedule of events, info about souvenirs, hotel, transportation, etc. with our mailings.