I've never done it, but it always sounds doable. We have raised hundreds of meat birds, from hatchery "heritage" birds, commercial lines of Cornish giants, excess cockerels from backyard breeders, and our own mutts. We've tried a number of grain mixes, commercial feeds, and scraps but never fodder specifically.
I'm assuming that by "fodder," you also mean the grain kernel. Some people are confused and feed only the green sprout part of the fodder to their chickens, which is not enough food. Don't forget that it helps to supplement pure grain or fodder feed with a vitamin and mineral mix. We like to use Fertrell poultry nutribalancer, but there are other options. We farm worms for extra winter protein, but have found that cooked beans and meats have been our easiest source of protein for our chickens.
Free range "heritage-type" chickens take about 16 weeks to reach 3-6lbs, depending on the breed. In my experience, it depends on what kind of stock you have. Hatchery stock will often be smaller and/or not to breed standards with growth rates. For your Cornish, I would weigh them at 15 weeks and see where they're at, and slaughter them whenever they reach 5lbs. I would not expect hatchery Dark Cornish to reach 5lbs by 15 weeks, especially if you're using mainly fodder, but I'd be interested to hear your results! Expect up to 24 weeks for the size you'd expect of a roasting bird (2-3lbs carcass weight). Remember that pullets will take longer to reach weight but will be rounder and have a more traditional carcass, whereas the cockerels will reach weight sooner but will have longer bodies, even on a Dark Cornish. Sometimes this allows you to slaughter the hens sooner for a perfect tiny round carcass, aka Cornish game hen

Have you eaten and cooked heritage-type chicken before?
I agree with luvmypets, there is a specific thread on BYC that deals with using fodder for meat birds that it a great read, though I can't find anything on there anymore.
I'm eager to hear about your project as I'd love a better way to raise excess cockerels for the table rather than throwing out scratch grains.