As long as the leather does not expose to harsh elements such as prolonged exposure to heat and water, you can care for it as little as once every 6 to 12 months. The oil on your hands will naturally condition the leather as you touch it. You don’t really have to worry about it if the leather still looks lustrous and glossy.
Here’s the guide on cleaning and conditioning:
To remove dirt and unwanted marks, apply a small amount of saddle soap and wipe it with a piece of cloth. Remember to use leather conditioner after cleaning to replenish the oil loss in leather. You can use Neatsfoot oil or any natural conditioner cream for conditioning. Simply apply a thin coat evenly, leave it for few hours, and rub it off with a cloth in circular motion. Please note that vegetable-tanned leather will usually darken slightly after conditioning.
As leather is porous, it can never be entirely waterproofed. In case if the leather is exposed to excessive water, dab it with a cloth and let it dry naturally; do not use direct heat. And then apply leather conditioner and leave it for few hours (preferably overnight) for absorption, and wipe off the excess.