Do I need to do any maintenance for a wood retaining wall?
If it was built correctly (designed with seasonal wood movement in mind) it shouldn't need anything done to it. Depending on how you would like it to look you might want to use a sealer on it. As for how long it will last that all depends on what was used to build it. If it was pressure treated construction lumber then I'd estimate 30+ years although the newer pressure treated formulas haven't been around that long so no one knows for sure how long it will last.