Lvling a new char from 1 to 86 can take 6-8 hours if you kmow what your doing and your pushing as a group, if your solo this might take 4-5 days.
Lvl from 86 to 92 can take 20 plus hours or grinding in a full party and so much more solo.
From 92 to 99 could take you a week or even months.
Now there also the farming aspect of the game, where a lot of new or even old players sometimes don't understand how to do proper.
There lvl 85 zones in hell, each champion mob in these zones is mlvl86 and unique mob is mlvl87
These zones can be farmed with 0 MF(magic find) to find more rune drops or with MF to have a chace to find items.
There boss farming that has a greater chance of rare and uniques, Baal can drop anything in the game so does Diablo iirc but other bosses has a loot table that lower.
There the secret cow level that lvl 82, with champ that are 83, the mob density makes this place great for farming.
Key farming for mini ubers.
Mini uber farming for organ drops.
Uber farming for torch charm.
And think, one class can't do all of this content (some can if geared to the max) and that why this game repayable by a big marging.