They are very sneaky and require a trained eye to spot them.
They can last/appear for weeks, but since it is too late already, there is not much that can be done.
If you go to week 7 in my diary "cannabis conundrums", I have posted pictures of hermie flowers and bananas too and this may give you an idea as to what to look for, (as well as many other interesting odd cannabis pictures).
Hope this helps, Organoman.