older calculators not in regular use - how often and how long to run them

[Rafa]

hmmm. I would rather not have to adopt meticulous regiments with spreadsheet and schedules, but i also would be a bit itchy leaving things to chance.

I resolved to rotating the 20s and the 35s at school for about 2 hours daily plus an hour of homework. so i guess that's sufficient of fixed regular use each week for each of the two calculators. I try to use the 28s daily for about 30 minutes of messing around.
A 27s is however making its way slowly across the atlantic and will be replacing the 20s.

I feel like i would rather sell the 20s and the 35s than maintaining this routine of rotation. But if turning these two on once a week for some minutes and not having to take out the batteries in between then i would happily keep them.

Can someone confirm whether or not this would be sufficient to keep the 20s and 35s in good health?