If you handle your leather often, the oils from your hands should generally help keep the leather conditioned, this applies more so to smaller items like wallets & keychains.
If your leather begins to look dried out though, it's probably time for cleaning & conditioning.
First make sure your leather is clean. You may need to wipe it down with a damp cloth to remove any dirt, dust or debris. Allow leather to fully dry, a few hours, overnight is even better.
Next, using Smith's Leather Balm, Mink Oil, or any non-silicon based conditioner, test a small spot on your leather so you can see the results. Almost guaranteed the leather will darken a bit after application.
Leather is porous and will absorb the moisture from the conditioner. The heavier the coat, the more your leather may darken.
Apply 1-2 thin coats, allow to fully dry and wipe any excess conditioner off your goods before use. Optionally you can lightly polish/buff with a horse hair brush or microfiber cloth.
Repeat as your leather starts to appear dry or once every 6 months - 1 years.