I had a customer tell me that Garmin's Lifetime Map Update meant 3 years. In theory this could be regarded as pretty much correct as electronic devices may only last that long on average, however Navman think it is at most 4 years. However, the Garmin excerpt below states exactly what they mean and if Garmin so chooses it could be even less.
Garmin Australia recently stated that they are moving from Sensis Whereis maps to Navteq maps. By the way, Navteq use to be the company and product,