if i was to do it from start to finish i would say a good 72hours, maybe more. But i like to take my time with them, cos if you rush then you end up slipping with the powerdrill and putting deep scratches in the metal which you have to flat out again. (I know from previous experience.)
In full i Beed blast the whole manifold to get it all clean, then flat the surface down with 120grit, then 320 grit, removing and smoothing off the welds, then polish it in 3 stages.
I am more then happy to do them for people at a cost, as its very time consuming and very messy.