when should I charge my iPhone to maintain it battery life and to maintain a healthy battery capacity?

Apple says you can charge anytime you like, but try not to let the battery drop to 10 percent or let the phone discharge completely before charging it up again.

i charge my phone when it falls to 20 and charge it up to 80 percent and my battery health is 100 percent after 7 months!

Hope this helps.

Whenever you like; the smart charging features of your phone will do what's best for battery health and life.