A Dremel can be used for that, but I'd not recommend it.
A Dremel is better for fine shaping and small projects - for something like fender trimming, I'd use an angle grinder, a Sawzall, or even a Jigsaw (if you use a saw, lay down a wide path of masking tape - makes your mark easier to see, and saves you scuffing up the paint with the saw "foot.")
You can use a jigsaw for the bulk of the trimming, and then use a Dremel to shape. I seriously doubt that it would take a week to ten days (then again, I do use a Dremel a lot...) but 2-3 hours per fender sounds about right, if you use a reinforced wheel (if you use the regular vitrified wheel, double that.)
If you use the grinder, bear a couple of things in mind:
1) Don't force the cut. A fairly light touch is still indicated (not as light as with a Dremel, but you still want to take it easy.)
2) Cutting sheet can be Hell on wheels - have some spares. Sheet presents a sharper edge to tear away vitrified abrasive than cutting something solid.
3) You may get tempted to use a "grinding" wheel to cut - don't. It won't work worth a damn.
