Well, torture won't pry any useful information out of the victim, because while he will undoubtedly eventually tell the truth, the torturer will not recognize it for such. There's way too much "noise" to the "signal" there, and the torturer will only hear what he wants to hear.
But given time, Gul Madred would obviously have gotten Picard's full cooperation. By that time, any information Picard could give would be outdated, but he would still be useful as a double agent (as long as Central Command recognized his fundamental untrustworthiness), a propaganda tool (although humans wouldn't be fooled by a human shouting pro-Cardassian slogans, some other species might be impressed), or at the very least a means of luring in further victims.
I doubt Madred was trying to get any of those things out of Picard, though. Cardassian torture would probably be based on a long tradition of extracting confessions out of the victims, and torture is always effective for that! It would suffice to have Picard admit to spying, theft, sodomizing of space sheep and other crimes against the Cardassian Union in a public broadcast, after which he could safely be executed. And Madred could enjoy the path to that destination, as he apparently had no better things to do.
Timo Saloniemi