The FW Cudas have a high metabolism, and they are not a 'glutton' fish with a bulky body, so I DOUBT they could live for 'months, easy' without eating.. Especially if they are juveniles. I wouldn't go more then 3-4 weeks at a time..
Xoluotun had some great info, and his method is AWESOME. Associating food with human presence is the key, definately.
There are some floating carnivore pellets that are soft, which I have heard alot of people have had sucess with.
Heres a trick I was told to use for pure preds:
First get them hungry, feeding only every other day.. So when you do release a single feeder fish into the water, they quickly snap it up. After that, use a freshly chopped worm, so it wiggles as it hits the water. Hopefully you can get them lunging at the worms as soon as they hit the water.. Then after that, they should just lung at about anything.. Offer frozen seafood, as it has taste and texture that manufactured foods do not..
Good Luck!