Unlike your other batteries most phones come today with a lithium ion or lithium ion battery(deviation is rare).
Those it doesnt have the life of the other batteries. Li is very versatile.
You can charge it when it's 70% or when it's 20%.
But still here a few rules to stick to if you want to get say a 4-6 month extension on the average life.
1. Charge when below 20%
2. Don't use(heavy usage) whilst charging.(optional won't cause much problem)
3. Charge for an extra 20 mins.
Eg: if your phone takes 2.5 hours to go from 18~100% then leave it in for an extra 20 mins so 2 hours and 50 mins
4. If you find it becoming too hot, remove the case or place in a cool room(basically don't let it get hot)
5. If not using for more than 1.5-3 months discharge to 50% and store in a cool place.