>>58837590It's fairly quick to evolve in its debut generation, you get it at Mt Pyre, at which point you can switch train it up into the early 30s in no time at all, and from there its a mere handful of levels, a gap you'll close before the next gym even.
It's in Sinnoh that it takes forever to evolve, like a lot of previous gen mons in gen 4, by virtue of being introduced earlier than in other games.