I bought a few Keepower and Tenergy brand 18650 cells 3 years ago and they seem fine. I use them infrequently, probably charging them back up every 6 to 12 months. The few times I use them they probably only get drained down 25%. How long will these last on average?
I’ve got well used 6 year old Panasonics. Still going strong.
They’re made for repeated charges e.g laptops. Normal ratings go to 300 cycles easy, but with lesser strain they last even longer.
You charge them at most twice yearly? With that usage, they’ll last way longer than becoming obsolete. 
High temperatures and high voltages will shorten the lifetime of lithium-ion batteries.
If it is charged to 4,0V it will last a lot longer than at 4,2V.
This might be interesting if you really want to know more about lithium-ion cells:
^^Nice vid. Thanks.