Adagio doesn't have a specifc bpm indication, it's the composer's idea of how the music should FEEL, and the same tempo marking can mean different things to different composers. For some composers, largo is slower than adagio. For others, it's the other way around.
So, the bpm you use is going to depend on a lot of differnt things. Mostly it's going to depend on you - whatever tempo lets you express the piece best is what you should use. Remember that great music is more than just being technically correct, but being expressive too.