The batteries are CR1616 if I remember right. Both mine and mife's have lasted 3 years with no prob at 2 journeys a day so certainly you will get 2 years evry with quite a bit of use.
You have to undo the little screw, pop the key shell apart & take out the plip module from inside the key then unclip the back from the module and the battery slips in to the back plate. It's actually quite easy once the screw is out.
Just aware that if the key battery fails you need to open the car with the key itself, which sets off the alarm and I don't think it stops until you disarm it with a button press = get the other key out ?
Also, would the transponder recognise the key if the battery was dead = stuck
The spare has never been used but the main key is used twice a day.
Lexus changed both mine at 2 years, is the Civic the same ?
Greedy is correct, the battery is a CR1616. Book of words doesn't give battery life but suggests that if it takes several pushes on the button to lock/unlock then its time to change. No other warning with regard to alarm etc..
For info, never changed the battery on my old Civic key and had that for 4 years...