I'm talking about linguistic maturity, here. Auxlang users may sit around all day wearing pajamas and picking their nose for all I care. I'm trying to gauge reasonable literary projects for the better-known auxlangs and their learners. Essentially, the breakdown will be something like this:
New user/auxlang: anecdotes, proverbs, etc. (a paragraph or so)
Stable beginner (basic grammar/vocabulary, nothing fancy): flash fiction (a page or two)
Intermediate (moving beyond basics into optional gimmicks): short stories and novellas (about 10-100 pages; may include abbreviated novels)
Mature (advanced gimmicks generally well established: odd word order, etc.): novels without major linguistic complications (not Tom Sawyer, for example)
Advanced (can handle most anything)
Now let's get controversial:
Esperanto: Mature to advanced. Active users (verkistoj) are often "mature."
Ido: Intermediate to mature. Primary problem is slow lexical growth--not a linguistic failing but a flaw in Idist culture. Theoretically resolvable.
Occidental: Intermediate. This is a user-base problem; Occ is capable of advanced usage, but it lacks coherency and user base.
Interlingua: Mature to advanced. The Latin/Romance schism is a problem, but it can also be harnessed for literary purposes.
Lingua Franca Nova: Intermediate at best. Syntactic issues and user-base problems.
Lingwa de Planeta: Stable beginner (more or less) working toward intermediate status.
Sambahsa: Anomalous; the language itself is probably advanced, but the user base (apart from Dr. Olivier Simon himself) is both small and ill-equipped. This puts it on a par with some revival projects, such as Idiom Neutral, but with a better corpus.
So if you're interested in producing novels (original or translated), your best bets are Esperanto and Interlingua, though Ido and perhaps Occidental are also worth considering. If you'd rather pioneer with shorter works, try one of the less mature auxlangs.