I use two rear lights; Thunderbolt and Hotshot. My daily ride is :55 - 1:00 a day, and I know my thunderbolt (I always use high blinking) will be dead sometime during the third ride, so I charge it after two. The hotshot (blinking) lasts 5 rides (just yesterday morning). I have not ridden it until it dies, so maybe I'll try #6 this morning. My back of the envelop bike log (below) keeps track of when to charge.

day, date, time, distance, calories, garmin 500 percent charged, th # of rides since charge, hs # of rides since charge.

br 97/98 ^ 100 br is before ride front 97 psi, rear 98 psi, topped off both to 100 psi

It looks better when I enter into my txt file using tabs to line up entries.

fr 10.09.15 :55:37 15.3 870 36 th 2 hs 5 .. th & hs chgd
th 10.08.15 :54:43 15.4 805 41 br 97/98 ^ 100 th 1 hs 4
we 10.07.15 :55:21 15.3 767 46 th #2 hs #3 , th chgd af
tu 10.06.15 :54:28 15.3 793 52 br 90/90 ^ 100 th #1 hs #2 rain yesterday
su 10.04.15 :59:55 15.4 753 58 th #3 hs #1 th dead..chgd ar, hs chgd br
sa 10.03.15 :56:03 15.3 766 64 br 97/97 ^ 100 th #2 hs #5
fr 10.02.15 :57:53 15.4 717 70 th #1 hs #4
th 10.01.15 :58:56 15.3 730 76 br 97/97 ^ 100 th #3 dead hs #3 .. th charged