Waiting 15 minutes is overkill...
My observations when using standard 4' single tube flourescents...
For paper (when printing), you can get to work immediately.
The afterglow is far too weak to fog paper. Period.
For film, it depends on film speed and ceiling height (lamp to film distance).
Wait one minute, and you can get right to work with slow (ISO 25, or so) films.
Wait two minutes for medium (120 speed) films.
Three minutes ought to do it for 400 speed films.
I have had standard 4' flourescents in my darkrooms for the past 50 years.
Quality of light is just as important as quality of dark, and after 3 minutes dark quality is top-notch.
Flourescents reduce shadows better than any other common lighting as far as I'm concerned.
Judging print quality under shadow-free ilumination is critical for my 74 year old eyes, and only flourescents fill the bill.
Reinhold
www.classicBWphoto.com