Well, there's no clear cut definition naman actually. I guess it all depends on your goals.
In general (for me at least) I think you get past the "beginner" stage when you know the fundamentals of proper form, rep speed, proper weight etc. When you establish the mind-muscle connection and you dont just go through the motion to finish a set or a cardio session.
You'll probably stay in the intermediate stage when you finally can determine approximately what routine(s) or exercises you need to do in order to see gains, whether you're trying to cut or bulk.
I think you can honestly be an "expert" when you know really deep knowledge stuff from how your body really works and stuff like that.
For me, I'm probably in the early stages of intermediate weightlifting, and (knowingly) staying in the beginner stage in cardio hehehe...
beginner to intermediate - when you have a legit split program. a goot mind to muscle connection. descent development on both fat loss and/or muscle size. (dito pa lang po ako.)
intermediate to advanced - trains 2x a day. excellent muscle definition bec of their low bf. excellent form and execution when lifting. follows a very strict diet, and training regimen.
hardcore to extreme - jumbo sized humanoids with comic book muscle definitions/mass. guys and gals that can bench press a mini cooper or squat a vw beetle. hehehehe. eto na siguro yung mga sumasali sa strongman competitions, or those prestigious Mr. whatever contests.
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"you can work out hard, and you can work out long, but you can't work out hard and long."
Well said Theveed. I agree. If you already know the proper form, then, you're not a beginner anymore. If you're in advance phase, its like makin your own workout, cardio & diet program.