First is the quantity issue. They don't really "fatten up" but they have a fairly fast growth rate until maybe 5 months of age. Optimum can vary by individual by a few weeks. He is not going to be as big as the store chicken. By the time he gets that old he could be crowing and trying to mate the pullets, quite disruptive.
The quality mainly depends in how you cook him, which is very age dependent. When his hormones kick in and he hits puberty (which should be soon if it hasn't already started) the meat gains texture and flavor. The additional texture means you have to cook him differently so the meat isn't tough. You don't fry or grill him but there are plenty of techniques to cook an older bird and get tender meat. The hormones also increase flavor. Some people don't like it, some of us do. It can be different from the store chicken because those are butchered before puberty hits.
Some people butcher dual purpose cockerels as young as 12 weeks. There is very little meat there but it is before puberty hits. Maybe they aren't supposed to have roosters or even chickens at all so they don't want crowing to give them away. Or maybe they don't like the behavioral changes at puberty. Many people seem to butcher at 16 weeks, that's typically the earliest you have a decent amount of meat. I don't want to wait after 23 weeks, the growth rate really drops after that. I bake mine.
Some of us find it is worth the effort to butcher but some do not and either give them away or sell them. As you can see different people have different opinions on how to get rid of a cockerel if they decide to. Some are more attached to theirs than other people. I don't know what the right decision is for you. We are all unique and your decision should be based on your goals and desires, not mine or anyone else's.
Some people raise Marans specifically to butcher them, they are a good dual purpose breed. As far as butchering a dual purpose cockerel a Marans is as good as any other dual purpose breed.
