Regardless of what others talk about Rushing, the main story is about 10-12 hours or so with the Episode missions as well.
From there it varies on how you want to spend your time. You'll have Episode missions (around 4), a huge amount of repetitive side missions (go here, scan this, report back) that are worth the same xp/rewards all the way through San Fran at the end. There's a few different side missions, like 8-10, that are series that reward better gear than the cut and paste side missions.
After that it's all about four things:
1. Farming contracts for rep. You'll need 1000's of rep on the vendor you want to gamble for mods with set bonuses on items. Since the mod you get will be random for weapon and may not even be the one you want it if it is it's going to be a huge rep grind on contracts.
2. Major Arkfalls. Minor Arkfalls leading up to it will drop white/greens and then majors are usually a green/blue/purple or two. Also the only way to get decent XP after you hit 40 Ego Skill Points. XP/level goes up about 2k per level so eventually you hit this wall of requiring 100k+ just to get another point (at 42 it's currently 62k).
3. Pursuits. There's a lot of Ego ranks to gain out of doing Pursuits and most will spread you out pretty thin on things to do. For example some pursuits require multiple weapon types/kills or ranks with certain weapons.
4. PvP. Not a fan of PvP in this game currently. It's all about the Cloak N Shotgun play which gets boring after a while. Entirely too fast and the amount of damage vs amount of HP you got is just stupidly balanced. Most TTKs are under a second which really plays up the whole whoever sees the other person first wins which just promotes Cloak N Shotgun play. I normally like PvP, but it's just too fast to enjoy it for my tastes.

