Took me around 3.5 years, although that was with a break of ~2.5yrs for a bit I'd got stuck on ("On a Rail" level).
At at guess I'd say it's around 20hrs of gameplay, but obviously if you've played it before then that would probably be reduced a bit as you won't be wandering around getting lost so much and will know the tricks to beat bosses.
As for the Graphics, there is in order:
-Original Halflife
-Updated hi-def models that came with Blue Shift
-Updated engine (HL: Source)
-Hi def textures etc in Black Mesa Source. NB this project is still a work in progress and like 80% of 'revitalisation' projects I doubt it will be seen through to completion, at least not in the near future.