My Labrador is just coming out of her second phase of chewing - luckily not quite as destructive as when her teeth initially came through when she was a young pup. She will however randomly chew on anything that just happens to be in her eyeline, favourites include kitchen cupboard doors, telephone and electric cables and when getting ready for a walk, her dog lead.
During her last teething phase I purchased this bitter spray and sprayed it around liberally on door surfaces, rugs, cables and her dog lead. Initially she was very interested in it as it was something new and interesting. But her little face looked quite pitiful when she tried to take her first munch on her previously favourite chew 'toys'. She was not impressed with the taste and drank a huge amount of water afterwards to get rid of it. She tried one more go at the doors and backed off - she has never been near them again.
I only realised she had started to chew again a few days ago when she proudly brought me a pair of old slippers which had big gaping holes in them. I have since been around respraying anything I think she might find irresistible with the bitter spray and so far I haven't noticed any more damage.
I have used this spray on a range of different surfaces and not noticed any damage from using it. However, I would be wary of using it on things such as expensive carpets without trying a test area first. I don't know for sure how long the spray lasts but once I have applied it to a surface I haven't needed to spray it again. I'm not sure if that is because the smell lingers or because my dog associates that item with not tasting nice any more.
The spray does smell quite strong and distinctive when you use it but it doesn't linger for too long.
So far one bottle has been enough for me to discourage my dog from chewing; there is still plenty of liquid left in the bottle so it does seem to go a long way.
A word of warning - wash your hands carefully after using it - if you accidentally get a taste of it yourself it really isn't very pleasant!