The borrowing of vocabulary is part and parcel of natural language evolution. Although Esperanto is now without doubt a full living language, it is nevertheless somewhat unusual as it is still spoken predominantly by second-language speakers and, although it does have its own associated culture, its culture necessarily draws on aspects from around the world. This being the case, how often is it that novel loan words - from whatever language - become widely accepted and commonplace and can thus be considered integrated rather than being a passing fad or idiolectal quirk? (cf. how I imagine the Danish concept of hygge will not be integrated into English but will soon go out of fashion)
This is a very good question – and a difficult one to answer! :-) Many Esperanto speakers are new to the language, and come to the community with a "basic package" of words (the most essential ones + all the affixes that can be used to construct new words). Then there are the fluent speakers who have a large vocabulary (and sometimes sigh at the amount of eternaj komencantoj – "eternal beginners"). In this group there are some people who are very quick to adopt new words (writers and intellectuals who want to expand the vocabulary for artistic reasons etc.) However, beginners keep coming to the language, and new words have to be explained afresh using the good old core vocabulary… A huge part of the really competent speakers – I’d say the majority – also have an ideal about ”keeping it simple” and actively oppose fancy new words (often lumped together as neologismoj). In short, I’d say the Esperanto community is quite conservative and new words enter the language at a much slower rate than they do, say, Spanish or English. And this is sustained by conscious idealism (”I want to keep the language simple so that everybody understands me”) and the structure of the language, which allows a lot of spontaneous word-formation through affixes.
That said, Esperanto certainly evolves! :-) The word for ”train”, for example, used to be vagonaro (literally ”an array of wagons”). Today, everybody uses the word trajno, which at some point was borrowed into the language. Although I’m unable to answer your question about how often (that is, the rate at which such new words enter the general language), I think there are various degrees of ”vocabulary integration” to consider:
- A group of Esperanto speakers use a word that is not fully Esperanticized – they don’t bother, since everyone present understands the word. For example, when meeting Esperantists in Spain, I’ve just been saying tapas in Spanish. A girl asked me if I would like to eat brunch-o (English sound + the Esperanto -o ending for nouns) – for a few seconds I thought she was talking about a bough (branĉo). Now, a lot of old-school Esperantists would call this sloppy behaviour, but in my experience, this happens quite a lot when speaking about local things and you don’t have a huge dictionary handy – just as it does when English-speakers use the Danish word hygge without properly integrating it into English (and with no intention to do so, since everybody feels it’s a fad).
- A larger group of Esperanto speakers use a word that is widely understood, but still not understood or accepted by the majority. Examples would be the words vlogo (vlog) and apo (app), that are understood by many young speakers – due to global Anglo-American Internet culture – but are not in the dictionaries. (My guess is that vlogo will soon disappear, whereas apo will become universally accepted – after all, even the oldest Esperantist has a smartphone these days.)
- A substantial part of the Esperanto speaking community use a word, that may or may not be in the dictionaries – for example blogo (blog) or mojosa (cool).
- Everybody uses a word that was not originally part of Esperanto: komputilo (computer), interreto (internet).
Then there are also these two interesting cases:
- a group of Esperantists use a word that is widely known inside this group, but not elsewhere. An example would be knufli (”to cuddle”) – I had no idea what this word meant until seeing it here in StackExchange; apparently it’s used by some Esperantists in France (and maybe a few other places).
- an author uses a word that only a few very well-read people know – still, it ends up in the dictionary. A big dictionary such as PIV is full of words that originally were proposals by some author. (For example glaŭka, a poetic word for ”blue-green”.) Maybe what really sets Esperanto apart from other living languages is the power that literary authors have had to influence the vocabulary – or at least the dictionaries! :-)
"Fully sanctioned" is an interesting way to put it. The Akademio does sanction words, but it's usually not the best measure of whether a word is fully accepted by the community. Another thing to keep in mind is that there are words that have been around for 100 years (e.g. hospitalo) that are still discussed to death and considered neologisms.
Another hidden assumption in your question is the idea that novel words will be loan words. One of the more notorious new words (mojosa) is not a loan word - but was formed intentionally from a contrived abbreviation.
It's worth considering whether it's true that Esperanto culture "necessarily draws on aspects from around the world." In my answers, I often remind people that an international language often will not have a term for a national concept (just as Chinese does not have words for uniquely American concepts, etc.)