I'm not an electrical engineer, but let's see if I can help shed some light.
We use a lithium-ion polymer battery, which boasts a better degradation rate than Lithium-ion.
The numbers we received on the battery:
Testing shows 80% retentive capacity remaining after 300 charge cycles (charging it from 2.75V to 4.2V). That's about 5 years @ 1 cycle per week.
Slap some conservatism on there (say 60%) and you're left with 3 years to get down to 80% retentive capacity.