For power consumption of the N900, see this table in the wiki:

http://wiki.maemo.org/N900_Hardware_Power_Consumption

Generally, it is better to not shutdown your phone when not using it, because reboot draws a significant amount of power.

But when you want to leave your phone off for months, then your phone will make this decision for you. Even when completely idle, the battery will be drained after a few days, and it will simply shutdown itself. Keeping it on continous power supply for months is not good for the lifetime of the LiIon battery.