I just bought a power bank for my phone, and gave it a quick test last night.
Charging a Samsung 9+ it took just under 90mins to charge 40% of the phone battery to full (started on 60%).
This particular power bank used 4% of its power per 10% of phone charge, so ended the charging still with 84% power left.
I did have an app running in the background during this time, and as i would normally put it into Aeroplane Mode to cut out all outside interference i do believe the charging time could be improved quite a bit, i will try it again soon and let you know, but i had to have the phone reachable yesterday as i was on call for work.
The power bank has 2 outputs, one @ 1A, and the other @ 2.1A which is the one i was using.
So on yesterdays evidence, seeing as my phone very rarely goes below 50% power in a day, i could theoretically get 5 days of charge out of it with no external power sources at all, although in the real world i would probably safely say 4 days.
Either way it's a bloody handy bit of kit for very little money. You can get bigger, faster and more expensive ones, but this was thin, lightweight and about the size of an average phone than can easily go in your pocket if necessary without feeling like you're carrying a brick about with you.
NOTE: It only comes with a USB to battery pack charger lead, so will need a USB Mains plug adapter to charge it. If you need to purchase one make sure you only buy one that puts out a minimum 2.1A, as anything less will take forever to charge.
And the outputs on the power bank are USB so you will need a USB cable to whatever connector your phone takes when charging it.
https://www.ebay.co.uk/itm/Ultrathin-50000mAh-LCD-Power-Bank-Portable-2USB-LED-Battery-Charger-For-iPhone-X/142745227181?ssPageName=STRK%3AMEBIDX%3AIT&var=441746815896&_trksid=p2057872.m2749.l2649NOTE: Buy at least the twin version.
https://www.ebay.co.uk/itm/UK-Mains-Wall-3-Pin-Plug-Adaptor-Charger-Power-4-USB-Ports-for-Phones-Tablets-CE/223476533247?ssPageName=STRK%3AMEBIDX%3AIT&var=522232631960&_trksid=p2057872.m2749.l2649