Much less common than when smoking was allowed on aircraft, thankfully. This article [1] about Air Canada flight 797, back in 1983, gives a good impression of h

Much less common than when smoking was allowed on aircraft, thankfully. This article [1] about Air Canada flight 797, back in 1983, gives a good impression of how fires in aircraft bathrooms were much more common back then, which led to pilots treating fires much more casually. [1]