+1

In other words, you want to minimize the amount of time your cells are away from 3.8V on average in order to maximize their lifespan. It's not a hard set rule as there are many other variables which affect the longevity of your cells too, best method I know how to check the health of your cells is to invest in a quality charger which includes an IR (internal resistance) meter. Not the best quality charger, but it includes an IR meter and will give you the basic information you need to check your cells:
Charsoon Antimater 10A Charger

It only offers precision to the nearest whole number where high quality cells tend to start around 1mΩ and will slowly increase over time to the point where they might reach around 8mΩ at about a year old which is the point where the pack starts to have a noticeable decrease in performance. Lower quality (budget) cells typically measure around 3mΩ when new and tend to degrade more quickly where they might reach 8mΩ in as little as 6 months in my experience with about a dozen different brands of budget cells that I have personally tested.

The quality of your charge leads will also effect your IR readings, if you decide to invest in a charger with an IR meter, then do yourself a favor and spend a few extra bucks here:
Trinity Charge Leads

If you want higher IR precision down to the tenth's, where new high quality cells might read around 0.6mΩ instead of 1mΩ and many more features then consider any of the iCharger brand chargers.

More info here:
How to prolong the life of LiPo cells