Someone may have mentioned a special fitting because some fittings have a perspex cover. U.V b doesn't penetrate glass or plastic, so if the fitting has a cover just remove it.
There is no way to tell when it has stopped emitting beneficial levels of u.v (as it decreases over time) unless you have an expensive uv meter, your particular globe and because of usage may last 18 months, or 6 months so people pick an average of when it will be no good.
Here is some testing on different tubes:
http://www.uvguide.co.uk/fluorescenttuberesults.htm 
