Forging Master took me some 30 hours to finish. Horrible, horrible trophy. You'll spend all your time fighting the same enemies over and over. For one of the items, there are only 3 easily accessible enemies who drop the items in the entire game! You just have to keep killing them and zoning out.Not to mention there are things like Eagle Eggs that would be absolutely ridiculous to get without the DLC (we're talking one egg an hour - you need three or four).
Yes, it's possible to finish in 6 hours. I think my final time for the Lightspeed was 6:30 or so, and I'm not all that fast.
I'm not big on Trophies, but if I really love the game and want to play more I'll spend some time on them, since they do (even if artificially) extend playtime. It wasn't worth it for Nier's Forging Master.