They'll chew until they're trained not to. You can either punish them if you catch them in the act or spray some bitter tasting spray on your furniture.
How about at -1? Seriously speaking, though, get him rawhide bones and teething bones and a bunch of toys to play and chew. Let him know that THOSE are his toys and gently show him those to play around. Always stress that the other stuff is not to be chewed, that the toys are HIS and that the non-chewables are forbidden. Let him know by a hard but firm "NO" and have him learn. Every now and then, it will happen. Don't get mad. It's just that he wants to play and doesn't know any better. Get him some plush toys your kids (or if you don't have kids, your neighbors' kids) don't want anymore, and toss them at him to fetch. His teeth are coming out and needing to be used.
Give him some toys to actually chew on. Put some pennies in an empty coke can, tape up the top. Anytime you catch him chewing on anything then scare him by shaking the can really loud. It'll scare him into stopping.
Puppies will chew when they are teething. If they chew when they are older, its because they need exercise to keep them from being so restless.
Man, you just have to teach lil dude right from wrong. It takes time, and is quite a process, but the more attention and time you spend working with him, the best results you will have. I do like the idea of posting pics of our pets!!
Probably a year or so. Our dog didn't quite tear up the house when she was a puppy and still doesn't, which is nice, lol.