I change the Wienn cells in my Canon FTb's, and all the other batteries in my cameras and other photographic equipment on my birthday, I know this is wasteful , but I never have problems with them failing.
I have heard that will add to the life of the battery, however I have switched to using the zinc hearing aid batteries with a rubber O-ring to adjust to the correct diameter. I can get a pack of 10 for the price of 3 wien cells. And they seem to last just about as long. Sometimes have to add a small piece of foil to get a good connection but that isn't an issue.
"There is something about the mystery
of what is on a roll of film that keeps
me shooting, none of that digital
instant gratification for me."
The washer from a Wein cell will pop off and then slip back over a 675 zinc air hearing aid battery. This works well for cameras which have an edge contact in the battery compartment. O-rings work well if it's a center contact battery compartment.
The exhaustion rate of zinc air batteries is also dependent on heat/cold and humidity. Under some conditions they last only a couple of months, under other conditions a year or more.
The only problem with standard zinc air cell batteries is that they are too big for my Honeywell Spotmatic.I found a great site by Microbattery.com which showes the actual dimensions of hearing aid batteries.Ordered 312 zinc air.