Loading nickel metal hydride battery for the first time I just bought a new hair clipper and I was about to start shaving my head when I noticed that it was almost at 0%. It has a nickel metal hydride battery. 
The manual says I have to load it for 24h hours before using it the first time or whenever it has been unused for a long time.

I started calculating. 24 hours are 1440 minutes. Dividing it by 100 makes it 14.4 minutes to gain 1%. To gain 20% it'll take 288 minutes (more than 4 hours).

Well I started to load it like an hour ago and it has almost 40% so far. That's not 4 hours. So it'll be at 100% in a few hours. I think in the evening it's done.

But the manual emphasized that it should be 24 hours ! So I wondered why do they want me to load it that long even when the percent-bar shows me it's fully loaded and ready to use? I also looked up some reviews where the customers wrote that the battery's lifetime is very short.

Would it cause a shorter battery-lifetime if I use it for like 10 up to 20 minutes in the evening (in about 6 hours)? Or more in general, is there something about the intrinsic chemistry of the battery which requires (in the most cases) so long before we use it the first time? Is that even necessary?
Battery on 0% usually still has some voltage, and battery on 100% usually can be overcharged. However, for many types of batteries it is not healthy, leading to premature degradation, the exact 0% and 100% charges as well as initial load procedure and storage requirements are marked at point that happens to prevent battery degradation and increase its capacity.

While I'm not aware of what exact process is intended to happen in Ni-hydride battery, I'm pretty aware that preparation procedure for battery MAY be required. For example, during initial cycles of lead accumulator, its capacity increases. The reason is in its chemistry, as it actually works on lead oxides transformations and the cycles increase layer of oxides on electrodes. 

Since Ni-hydride cell includes electrode with half-reaction 

$\ce{Ni(OH)2 + OH- = NiO(OH) + H2O + e-}$

I guess, that during initial charge the manufacturer expects that layer of nickel hydroxide is formed on positive electrode.  But this is just a guess, you'll have to search specific info on this particular type of battery to be sure. Ni-hydride batteries are resistant to overcharge at low currents, so if you use appropriate smart charger, say, shipped with the batteries, the overcharge is safe, so, since the manufacturer requires it, you should follow the manual.