That charge rate assumes 100% efficiency (which is not the real world). For NiMH in particular, multiply the mAH rating of the pack by 1.4 then divide by the charger's output.

So 1200*1.4=1680. 1680/50=33.6 hours for a fully depleted pack. If you have a 120mHA wallwart charger then 1680/120=14 hours. The 120mAH wallwart charger would be a C/10 rate for the 1200mAH pack. 50mAH will work but takes a lot longer to "fill up" the pack.