You will love Bilbao, I am not going to be ready to leave this awesome place in less than a month
. If you have time while in Bilbao, take the metro to some of the surrounding areas outside the city. There is a lot more to the city than just the Guggenheim. Try taking the metro to Sopelana beach (metro: Sopelana, or Larrabastera), explore Las Arenas (metro: Areeta), Plentzia (metro: Plentzia). If you are into shopping, try either Casco (metro: Casco Viejo), or Gran Via (metro: Moyua)
Also, while in the north, dont miss San Sebastion, Santander, and if you have a chance, try the small town of Santillana del Mar.
Enjoy Bilbao!