Not only would it take a long time, I imagine the results would be extremely random to the point it would be unreliable as an execution method. Carcinogens, and the way each person's body reacts to them, are highly unpredictable. That's why health disclaimers on carcinogenic products say "may cause cancer" and not "will cause cancer."

It would be far easier to just shoot or stab the target and make it look like a robbery gone bad or something.

Reminds me of that scene from Austin Powers where they parody those intricate death traps in James Bond films...

Dr. Evil: I'm going to kill Mr. Powers by putting him in an overly elaborate and easily escapable contraption and assume it all went according to plan.

Scott: Why don't you just get a gun and shoot him? He's right there!