Basically, Lithium Ion batteries generally have a shelf life of 500 full charge and discharge cycles (on average) so given sensible use, it should last a year and a half before it starts to perform really poorly. It's also worth noting, they will gradually lose capacity over time anyway regardless of how they are charged.
Full discharges aren't recommended, but they're not really damaging in any sense. I tend to charge mine (if I can) when it gets to around 40%, but I never wait till it won't switch on. Also, whilst the phone reports charging is completed, it won't be, so leave it on for an hour or so after it's claimed to have a full charge.
Li Ion batteries also don't have a memory so little top ups here and there won't do it any harm at all.
Finally, it's not dangerous, but again recommended to switch off/not use whilst charging as leaving on can sometimes "confuse" the charging chip in the battery.
If you want more info, check the link below -
How to Prolong Lithium-based Batteries – Battery University