There is a setting in the App that lets you choose how many days before discharge it ranges between 3 and 10 days I think . Mine is set at ten but I will normally fly at least once within 10 days then recharge and pack away again. So I guess 10 days is ok ? They haven't discharged yet . I've only had my P4 for a few months so a discharge is on the cards soon going off battery maintenance that I've read about.
Hope this helps
Safely ? If that is to do with fire risk - then indeterminate, as the risk of incident increases with them being fully charged. Left fully charged and with for example a hot summers day - they can puff up and in extreme - puncture / then self combust.
That's the paranoia bit out of the way.
The batterys have an auto discharge function built in that users can set as they wish. The P3 series has 1 to 10 days setting.
Personally 10 days is too long as damage in lipo's is accumulative. Each long charged cycle will add a little more resistance damage ... till you find your packs don't perform well. Personally I like 3 days auto setting.
Next point - when the battery's sit - don't check state - this resets the discharge countdown. If you are going to fly - then even if you think battery is OK - switch it on and put on charger yo make sure its full. Why switch on ? If you don't and battery is only partially discharged - it will not charge.