This isn't 1993, you don't need to charge batteries for 10 hours or whatever anymore. Just charge it as you need to.
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
Yes, next time you buy a phone, read the manual. The manual for any modern phone that uses Lithium Ion cells will tell you to charge it until the phone says the battery is full.
Follow what the manual says, in case of a bad battery, that will hold up, not what some idiot sales person says.
In fact, from my understanding, it's BAD to leave a lithium ion battery on constant charge, one of the reasons why laptop batteries get old and lose capacity so quickly.