Writing a device driver can be pretty simple, or it can be almost arbitrarily complicated. For instance, I've been involved in a project where it took six of us almost three years to solve ONE bug in a device driver. Of course, we cleared out dozens of other bugs while looking for it... the code improved immensely. The fix turned out to be an eight line patch, that cost, conservatively, about a million dollars.
But, as a side project to that, I wrote an ethernet driver from the chip data sheet in a week, and took another week to debug it. Haven't needed to touch it since.
There's no way to say in general how much work a driver will be; a GPU driver could cost hundreds of millions, a driver for a single LED costs a couple of hours work at the most.
