GoKawiilTo recap, if you're storing a phone, power it down when the battery is at about 50 percent and charge it back up to this point at least every six months. Most importantly, don't throw it in a drawer completely dead nor fully charged. Don't forget to store it in a cool, dry place with temperatures below 90°F.
All that said, leaving a phone lying around for months isn't a total death sentence. I have left some of the best Android phones released over the last five years powered down for months and charged them for comparative reviews. They work fine even if the battery capacity isn't as good. I even recently found an old BlackBerry PlayBook tablet that hadn't been booted up since 2011. I plugged it in and it successfully charged and works! But it's clear the battery life has lessened as it barely lasts a day before I need to recharge it.
The bottom line is that if you power down your phone, it won't have the same charge state when you power it back up, especially if weeks or months have gone by. It might still show the same battery percentage if you've only powered it down for a 12-hour flight, but the battery is still draining through self-discharge and the clock-and-wake circuit, albeit so slowly you won't notice after such a short period of time. However long you turn off the phone, powering down doesn't mean the phone's battery is completely on pause until you use it again. But it can help conserve what battery life you have left in a pinch.