I did a battery change on the Suunto T3 about 2 years ago. Looks like that is how long the battery will last in the watch (without use). Not sure how long it will last with use. Here is the link to the last battery change.
Here are some updated photographs of the process. The process would be the same. Just remember to check that the battery works and the light works before you close up. Oh, yes, and also do not forget to lubricate all gaskets.