About how long does it take to raise meat chickens?

If I'm not mistaken the Cornish X is the fastest growing of the meat birds.  They are ready to butcher in 8 weeks.  You can let them go a little longer, but unless you take great care of them they are prone to health problems like heart attacks and broken legs from the rapid growth.  Also, after that time you are not getting much more meat compared to the amount of feed they are consuming.

I am gonna stick my neck out a bit here, and someone correct me if I am wrong.
   A hybrid bird is created by breeding two specific breeds to take advantage of the favorable traits in each breed.
  A breed or something that is genetically engineered has been messed with on the molecular level, meaning that the genes are taken from the reproductive cells of each animal and certain genes are used to produce an offspring that is completely different from either parent donor.
  My own thought is that this is what happens when animals or plants are crossbred for many generations to reach a desired type of offspring, but the engineering part just skips over all the generations. The danger there is that you can get unexpected results that are undesirable that would have been caught and eliminated early on the old fashioned way..........

They are just products of selective breeding not genetically engineered.  You could definitely raise dual purpose birds, but they take longer to grow out, and do not convert their food to meat efficiently--meaning that it will cost alot more in feed to produce a less than satifactory chicken "meal".  There are red rangers, that are popular too, but the cornish x are definitely more cost effective, give you a nice roasting bird in a short amount of time.

Selectively breed, but yeah they're ugly (tasty but ugly).  In 5 months you could get a dual purpose breed like Rhode Island Reds or Plymoth rocks to butchering size, but It would take about 4 months to get the same sized birds.  So do you want to put in 2 months or 4 in doing this?  also how many can you manage at once?  a 5 month timeline would allow 2 sets of Cornish X in the same time as 1 batch of RIR.  As is cornish X is what you already buy at the market, unless you specifically buy hertiage birds from a speciality market.

I opted to go with the basic "Dual Purpose" birds for my first time.  I am not sure I wanted the rapid growth of the Cornish X as well as the concentrated poo (i.e., 16+ weeks worth in 8 weeks) issues that I have read about.  Might talk a bit more feed, maybe not as they enjoy free ranging during the day.

just my two cents (little experience as it has)

Give the cornish a try. In 8 weeks you have these huge plump juicy birds. I was hesitant too at first but after trying them compared to the "regular" roos we butcher these made excellent roasters. We used a portable chicken tractor to keep them moved around the pasture. They do stink but man they sure do taste good after all is said and done. Some people add a little lime over the area where the birds are after they are moved to control the smell; it does work. Good luck!

I keep reading about all the problems folks have with CX - lazy, just lay around and eat, heart and leg problems, etc.  I haven't been at this very long but I've not really had all those problems.  The CX I've raised have been known to scratch, try and fly, jump up on the top of the waterer, stroll around the meadow in the afternoon, bump chests, eat bugs and grass, and all of the other things that chickens do.  They are certainly not as active as my friends' layers but they don't ever get that old to learn these things. 

Has anyone raised CX's side by side with adult layers to see if they learn this behavior that way?

Yes, they already know how to do all those things.They are chickens after all; just alot bigger.They just prefer to eat more than the average farm chicken. It is called farm obesity.