If the battery is completely discharged then at C/20 it will take 20 hours plus 40% more for losses. The total time to make the battery fully charged is 28 hours.
Energizer shows that a high current charge at a C rate makes a Ni-MH cell fully charged at 110% of its capacity.
A C/10 rate makes a cell fully charged at 140% of its capacity.
Maybe C/20 needs 150%.