Check out this Paragon Level Chart submitted by Kimano
Description: 60% 'weak' mobs, 30% 'medium', 10% 'hard', 3% of mobs are elites/champs.
There are several columns for over, average and undergeared. They change the mobs/min to 40, 20 and 10 respectively.
I checked the spreadsheet against the current highest paragon levels for softcore, and got reasonable values.
(~40 hours of play time gets you 36), which gets you a ~60% playtime since launch of 1.0.4.)