Short answer: As much as you'll give it.
Long answer...
It'll probably take between 40 and 60 hours /played to make it to 60.
Rift end-game is generally considered to be PvE. I've never been a cutting edge raider, and through Chocolate was a good tier or two behind progression. As content gets older, it's easy to PUG. Other than PvP (Warfronts and Conquest) and progression raiding, you can:
- Do instant adventures.
- Do zone events.
- Do completionist things like Carnages, achievement hunting, artifact collection.
- Make your dimension a masterpiece.
- Standard crafting, including fishing, cooking, and the likes.
- Hunt Rift progression.
- Crifting.
- Limited AH, if you're into playing futures markets, sorta.
- 5-man instances (random and not).
The Rift design ethos is "Log in and start playing" - so, as long as you're not too picky, you can almost always be teleported to something to do - log in and be killing stuff within less than 2 minutes.