IMO there is no true one-time leather protection. Leather over time needs care and maintenance. Whether its once a month or once every 6 months, it depends on what it is exposed to. Typically if its exposed to the hot sun often daily, then it will probably need more cleaning and conditioning. But if its garaged then it probably only needs minor maintenance.
The worst thing that can happen is hardening in which at that point will be hard to bring back to life.
In my previous '02 Acura RSX Type-S, after 6 years of use it was still soft and supple and all I used was over the counter leather cleaners like Meguiars leather conditioner.
2015 Diamond White Pearl MDX - SH-AWD