I have a Garmin Oregon 300. I find that the regular Duracell AA batteries last a couple of caching trips, i.e. perhaps max 8 hours. The Duracell Ultra Power batteries last notably longer than that. After the initial 'Low battery' message, I find they don't run out straight away.
It may save money in the long-term to purchase rechargeable batteries. I'm not sure how long these last, or exactly how they work, since I (for some reason) have never got round to buying these
Perhaps someone else can answer this?
Hope I answered your question though, and happy birthday by the way. Enjoy your GPS