How long should it take for a trickle charge to charge a Doorbell 2 from 30% to 90%

I helped a friend install a Doorbell 2 and the battery was not charging. I redid the wiring and it's now charging. We didn't have time to charge the doorbell using the USB cable so we installed it at 30% and figured that the hardwire would trickle charge it back up to 90% or better. In 20 hours, it's gone from 30% to 37%. The temps have been moderate (48-70F) in the last 20 hours and she is not in a high motion area so it shouldn't be struggling because of that. Does this sound about right? If it's linear, at this rate, it will take about 6.5 days to charge up to 90%. Does that sound reasonable?