Broken Steel is by far the best add-on you can buy, in terms of importance:
- Broken Steel
- Point Lookout
- The Pitt
- Operation Anchorage
- Mothership Zeta
Broken Steel itself adds on a good amount of time just on the leveling aspect and the hunting around the wasteland, and that's before the missions. Point Lookout is alot more story oriented, but it's bloody hard as hell, and found the length came from the sheer difficulty of it. The Pitt is fun, and if you play it out fully can last a couple hours. Operation Anchorage is short, but the goodies are the real treat, maybe an hour or two tops. Mothership Zeta isn't that important, it lasts a little longer than Anchorage, but it isn't in any way important or necessary.
Broken Steel- 5-7 hours on leveling and story combined
Point Lookout- 3-4 hours of pain
The Pitt- 2-4 hours depending on how OCD you are
Operation Anchorage- 2 hours, it's hard not to beat this in the exact time, you will see what I mean if you play it
Mothership Zeta- 2-3 hours, but it's really boring and tangential in relation to the story, not really worth it.
Hope it helps.