Assuming this isn't a form of Pica ( [
link to en.wikipedia.org] the cat should grow out of it in about 2 years time (around age 2).
If it is a form of pica and you leave it be, I fear the cat will start eating things it really shouldn't outta. While dogs are well known for pica behaviors (specially poo), cats are often not diagnosed properly due to this 'suckling' behavior at a younger age.
If you rule out Pica (blood tests and vet visit) then this is suckling behavior - its sort of like the kid who continues to suck his thumb.
Screaming at the kid to stop only makes it worse - beating the cat to make it stop will only make it worse.
This is a psychological issue caused by largely abandonment/separation issues.
Cats that have not been socialized, or who lost their mum too early will go on to be an adult cat that 'suckles'. Cats that have been beaten or traumatized will also suckle - its a comforting thing for them to do - like the kid sucking on his thumb.
This cat most likely has abandonment issues. If its worse when you get home from work/school you can be certain that you will have a suckling cat for life.
One cure is to have two cats - two cats in a home tend to satisfy the cats need for some social engagement and will remind it that its not alone. Cats, dogs and various other creatures we humans have adopted as pets are actually social creatures, that adapt well to entering our social matrix and do suffer being alone as much, if not more than, humans.
Edit Addition:
All of these people telling you to slap your hands, rattle stuff, spray the cat - they are telling you to psychologically abuse the cat. These methods don't work to treat the underlying causes of suckling. Sure the cat may stop suckling, but come up with other interesting behaviors - pissing in your shoes, clawing up the furniture.