I guess it depends on what you want to get out of the game. If you just want to do the MQ and a few side quests, then I'm sure you'll never come close to max level or deep perk builds.
Many of us plan to explore literally every square inch of the game, get every achievement, put in 300 hours, then do it all over again every time some DLC comes out. Doing that, it's likely we'll go beyond level 50 even, and with that much time invested, even 49 perks may not feel like enough.
Actually youll hit level 75 far before the 300 hour mark.
Level 50 comes surprisingly quick. Leveling is fast. Those doing "just" the main quest and the side quests they encounter on the road will likely finish around 50.
For an average player, I'd say level 50 after 30 hours, level 75 after 100 hours.
As usual, "slower skill growth" mods will be critical for those of us who like taking things slow and playing 300 hours.