Only the initial charge needs be very long. After that just charge and
discharge normally. This site is authoritative:
"Battery manufacturers recommend that new batteries be slow-charged for
16–24 hours before use. A slow charge brings all cells in a battery
pack to an equal charge level. This is important because each cell
within the nickel-cadmium battery may have self-discharged at its own
rate. Furthermore, during long storage the electrolyte tends to
gravitate to the bottom of the cell and the initial slow charge helps
in the redistribution to eliminate dry spots on the separator.
Battery manufacturers do not fully format nickel- and lead-based
batteries before shipment. The cells reach optimal performance after
priming that involves several charge/discharge cycles. This is part of
normal use; it can also be done with a battery analyzer. Quality cells
are known to perform to full specifications after only 5–7 cycles;
others may take 50–100 cycles.
https://batteryuniversity.com/learn/article/charging_nickel_based_batteries