I wouldn't be too much worried about discharge. Put a reminder to charge it once per year and you'll be fine. If after that year they still have nearly full charge, you can put the next reminder after 2 years.
All of the new USB batteries are lithium ion. There are two ways a lithium ion battery could discharge:
- Self-discharge. However, that's very minimal. Unlike non-LSD NiMH and lead acid, lithium ion batteries self-discharge very slowly.
- Soft off power usage. This is for the case where the battery device doesn't have a mechanical on/off switch but rather a circuit that's always powered on and then a push button that merely sends a signal to the circuit to turn the device on.
Out of these two, (2) is the most likely cause of lithium ion battery losing charge, since self-discharge isn't really an issue with lithium ion.
I recently purchased an Ecoflow power station (210 Wh, 300 W true sine wave AC). I set its charge limit to 95% so it never charges above this SoC to prevent rapid full charge degradation. This charging was a week ago. Today, I turned the device on and it still has 95%. So I can conclude it has lost less than 1% of charge, and therefore the soft off power usage is so small that the device will have charge for at least 1.8 years if not more. (This device doesn't have a mechanical power switch but rather a soft off circuit.)
However, it isn't entirely impossible for a poorly designed soft off circuit to rapidly discharge a lithium ion battery. I bought a beard trimmer that I use every 2-3 months. Annoyingly, if I charge it to full, use it, and wait 2-3 months, then the battery is completely empty and I need to charge it again.
There's no reason for a soft off circuit to consume so much power that recharge every 2-3 months is required. However, this beard trimmer is very poorly designed. I have several flashlights with soft off circuit and they consume so little idle power they retain the charge for at least years if not decades.
If your USB battery has a state of charge indicator, you can do an experiment: charge it, wait for a while, then see what charge it has left. If the indicator is percentage (0-100%) then you will very quickly get an estimate for how many years the charge is retained. If it's only 4 or 5 bar indicator, then obviously getting an estimate may require months if not years.
None of my USB power stations have high soft off power usage. I have several USB power stations (USB only, not AC like the Ecoflow) that also retain the charge for very very long time.
According to Wikipedia, lithium ion has 2-3% self discharge per month. If that's accurate, then it would take a year and half before the self discharge would be a serious issue.