It all actually depends on the number of Pokemon you have in your team, as you progress through the game. I played through I think Pearl in a Mono Type Duo Challenge, (I chose Fire types and used Typhlosion and Houndoom) and by the time I got to the Elite Four, I believe I was about Level 70; maybe a little less...
I'll assume that most people train at least 5 Pokemon; with that number of Pokemon in your team of which you actually plan on using, its rather likely that you'll be a bit under-levelled upon arriving at the Elite Four. A lot of the time, I usually get there at around Level 55+, but only after grinding up a few levels, beforehand.
At completion in D/P, I had Pokemon at Level 57 in the past, I believe. I can't recall what my team was like in my Japanese Platinum play-through...