When to charge the smartphones battery

I have a question regarding when to charge the smartphones battery
Some people say the charge and discharge cycle should be completed in order to maintain the performance of the battery
and i only charge my phones when it says power is low (15% battery - GS6) and charge it until its full

and if i have to charge again i will drain all the battery until its 15% again.. and then will charge it again.

what do u suggest ?

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