It happens more often that it does not happen.
50.000001% and over
There is no official percentage. It just happens more often than it does not happen. So at least 50.000001%.
As pointed out in the comments, often can be any percentage that implies somewhat frequently - and depends on the circumstances.
You may note that the person being quoted was characterized as speaking "tartly", which means that their remark is purposefully unkind, angry, or bitter.
"It's not unheard of" is often used in a wry, tart, or sarcastic manner to make it clear that what you're speaking about is actually quite common and you are in fact pointing out that your target's lack of whatever is bothersome to you. For example:
Woman puts on a brand new dress for a night out with her boyfriend.
Boyfriend: "New dress?"
Woman: "Yes!"
Boyfriend: "Thought so."
Woman waits a moment, expecting boyfriend to express appreciation of her new dress.
Boyfriend says nothing else.
Woman, angrily: "You know, it's not unheard of to compliment a lady who's wearing a hot new dress for a night out!"
(It can also be used sincerely, in which case the intonations will be very different.)
In general, "It's not unheard of" is a perfectly decent turn of phrase and need not be avoided for fear of sounding awkward or unusual.
Best Answer
Grammatically, it is perfectly valid to say something is "very much xxx", where xxx is an adjective.
Semantically, it is a superlative - intepretation depends on what the proposition is that the statement "it is very much true" is referring to.
Some propositions are either true or false - e.g. today is Saturday - there's not much room for this to be only "partly" true, so equally it wouldn't make sense to say it is "very much true".
There are, however, plenty of propositions which may legitimately be argued to be only partly true - e.g. it is a cold day today - the truth or falseness of this statement depend partly on the actual weather, and partly on the perceptions of those complaining about it.
An Alaskan might say "false, it's not cold today", to which a Floridian might respond, "what? no! it's very much true!"