Depends on the part - how much time. This for machining, or do you want a pretty rendering? What format do you want the model?
In the process of learning 3D rendering, complete novice so no help to you though; watching this thread anyway, maybe post up your project for us newbies. Mike
Modeling the part is the (relatively) easy part, if you have one to hand, or a pretty clear concept. Dimensioning and tolerancing said part so it can be manufactured, and will work in the application, is the tricky part.