You should completely drain your battery once a month
“For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).”
“Apple does not recommend leaving your portable plugged in all the time.” It adds that, if you don’t drain and recharge your Mac laptop in the course of normal usage, “Apple recommends charging and discharging its battery at least once per month.”
Of course, many iPhone users find that it’s easy to completely run the battery down…