It depends on the type of map. AVA maps can have lots of extra hours of work put into new terrain textures or special objects. MA maps generally do not have that but they do have hundreds of objects like bases, strats, towns, CVs, spawn points, shore batteries, roads and railways that all have to be placed by hand. These then have to be checked for properties bugs, runway clearances, SB firing angles, SP routes to the fields and so on.
It took me four years to finish CraterMA, but I had to re-write it a few times for compatibility with newer versions of AH. Also that was just doing a few weeks here and there in-between skin projects. I don't know how many hours it took in total.
I've made a rough outline of a new MA map which I just sent to HTC for their feedback. If they approve it I'll make a start on it. However skinning projects and updating CraterMA will take priority so it will be months in the making.