Right, the way map updates are priced it seems best to me to update roughly once every five years or so. My habit is to try the car nav first (because there are some things about it that are more convenient, especially the next turn information on mine below the speedometer) and if it doesn't find an address or the route looks somewhat off switch to my phone.
Having worked for a company selling satnavs and mapping, typically an automotive company has some number of map updates that will be released for a given product per year. It generally ranges anywhere from 4 to 1/year. However, there were some instances where it was a single update at the time of release and then no more.
