"it could take five minutes" is pretty limp, man. C'mon. "He just got his ass whipped pretty badly, and our star-destroying solar system killer is going to blow up - let's get him in there for that five minute Jazzercise course that turns him into uber-Vader."
There's more to these villains than just boilerplate super-competency and cold efficiency. They're sloppy, overeager, ambitious maniacs. They're radicals (they're fuckin radicals kill people burn shit fuck school), and there's more to them than just "I got everything planned to the nth degree"
They're chaotic.
Lol I know I was being flip about that. But really, what his "completion" entails is totally undefined. If we wanna say Ren isn't up to his potential -- and I think we should say that -- then we shouldn't put stock by Snoke's assessment of his "training's" "completeness". We should say things like "well Kylo is sloppy and over-ambitious, he's maybe never been really tested against another Force user". Or "Kylo has a real weak clarity of purpose, he seems to have desires to follow in Vader's steps but he waffles on the fortitude to commit".
"Training" is such a weird way to capture those ideas. "Training" connotes practice and drills. Kylo's got tons of practice Force choking fools and Force-freezing them at least.