RuuviTag comes with a CR2477 battery that theoretically lasts about 4 years.
However, in practice, battery lifetime will be somewhat shorter so to be safe, we tend to say it has a 2+ year battery.
Battery life depends on temperature. In cold temperatures, lifetime will be shorted due to battery and in really warm temperatures electronics consume a bit more power than on room temperatures. This can shorten the projected battery life.
See how to change the battery here.