...by which I mean not "how hard it is to pronounce" (challenging) or "how hard/complex the grammar is" (varies, for a conlang), but something more on the order of "how much language there is to learn"?
So I did a quick analysis of The Klingon Dictionary. The heart of the Klingon language is in the morphology, the system of affixes that derive and inflect the root words of the language. As for the affixes, there are 21 noun suffixes that fit into five slots, 32-35 pronominal verb prefixes, and 31 verb suffixes that fit into nine slots. Almost half of the total suffixes mark person in some way. Additionally, the grammatical sketch introduces seven dozen or so roots, almost a dozen stand-alone pronouns, a dozen or so numbers (as well as two suffixes for numbers), almost a dozen conjunctions, a half-dozen question words, and three dozen or so adverbials and exclamations. So you could probably fit a very solid working knowledge of Klingon onto about 230 flash cards. That's large but manageable.
As for the dictionary, I estimate that there are about 1,460 words in the Klingon-English section. With the morphology, that's a lot of expressive power, although the vocabulary intentionally doesn't cover a lot of semantic space.