Beirut itself is not all that interesting - 2 days maybe?
But there is so much more to see in Lebanon if you get out of Beirut.

We had a car in Oman, and drove around for 5 days, that seemed just enough.
Oman is very easy to drive around for a foreigner, I wouldnt dare driving in Beirut, its madness out there

The nightlife will be much better in Beirut vs non-existant in Oman.