Time consuming, but a great project if your a do-it-yourselfer.
I have done two. If you store your boat in the garage, then you could do it just fine over the winter months. Make sure you do a lot of research on the glue. Also, if your using outdoor carpet glue, you will need LOTS of clamps...my recommendation is get a lot of large binder clips and some thin 1/4" plywood strips. this will help you when you need to glue the lids. Also, Lots of blogs out there recommending Contact Cement for the lids instead of the glue. Not sure about that I have never used contact cement, however I like the idea of a quicker set time. The contact cement will set almost instantly where the glue will take up to 72+ hours depending on temp and humidity.
If it is an older boat and your not too worried about it, the carpet at Lowes/Home Depot will work fine. It is 16oz. If your looking for a very durable longer lasting carpet your going to want a 20oz non-looped carpet. it will last twice as long, but your going to pay double for it.
Also, if you plan on using the carpet glue, make sure it is OUTDOOR glue and read the instructions well. The Multi-Purpose glue will come up the first time it gets wet - hard lesson.
jWest - great tip.
Hendo - Large bottle of Morgans + the case of beer. +1 on the "get more carpet than you need." You will be surprised how much carpet gets eaten up by the box lids.
Good Luck and have fun with it.