i need to charge my phone when i start getting the empty battery beeping... the lowest green level on the desktop indicator might last 5 min as it may last 2 hours... it depends on the usage at the time... its "proportional" to the use u make of ur phone...

Nightly charging is probably best, unless you need to sooner with heavy use. Don't discharge more than necessary, as it is not required with Li-Ion (and could possibly be detrimental). I personally have yet to get a low battery indication - even with battery.charge_level.percentage at 1% and battery.reporting.current in the low double digits (although this was in the first few drain/charge cycles so calibration was probably somewhat off).