The last time Eyjafjallajokull erupted (in December 1821), the volcanic activity lasted for 14 months.
The latest press release from the Met Office states that there's no sign of the present activity coming to an end:
http://www.metoffice..../2010/pr20100511.html
Stromboli, off the coast of Sicily, has been continually erupting (to greater and lesser degrees) for over 20,000 years so, if Eyjafjallajokull follows the same pattern of activity, you might have a VERY long wait for your holiday!
Chris