Maximizing Lithium Ion Battery in low drain devices

I have a bunch of LiIon batteries. Some 18650 and some 10440 - I've converted a few things with them.

One item is a large LCD clock I have with 5 inch numbers. It previously took 4AA batteries... After some work with the Dremel I stuck an 18650 in it's back. Soldered some short leads to where the AA batteries connected.

I want this clock and the other items around my house (remotes, other clocks, whatever else) to last 10-20 years before battery replacement if possible.

I'm wondering which range of SOC I should be keeping the LiIon to achieve this.

The clock has gone from 3.440V to 3.395V in the past month. I haven't paid the closest attention to the discharge behaviour of lithium batteries under 3.5 volts, past it's usually much faster than the 4.2-3.6 volt range. This leads me to believe that I could charge this clock once per year to 50% and I'd have energy to spare. Is this the best thing to do though? Would I get better longevity charging it to 25% twice per year? If the battery is between 3.1 and 3.5 volts for a long time, is this worse for it than being between 3.5 and 3.7? The clock is clearly visible down to about 3.00, perfect down to 3.15, gone by 2.5, so I can use any of its normal range.