jtr is right.
The current bunch on comand training courses have all been in CX about 10 yrs. ...and they joined as F/O's. Add 2-3 yrs if you join as an S/O through the cadet scheme. May be a bit less if you join as an S/O with a few thousand hours.
This should reduce to about 7-8 yrs over the next few years as there is a big group retiring between now and 2005.