the job expands to fill the time allowed...saying that, there is never really enough time when working in production so the simple answer is you try to do the best job you can in the allotted time given.
The Art Director/Supervisor really dictates how long you have to concept.
For you own personal projects, taking the time to fully realize the design is always worth the effort as the more you know about the finished article the easier it will be the model with the finish result in mind but again if you have a time constraint you have to balance your time ( design verses modeling) to suit your own workflow.
There is really no substitute for practice practice practice with Maya,you might find that sometime in future you can model from less detailed references and that is something that is important to get used to since in production you might not always have the best reference or fully realized concept and simply have to create from imagination.