It is colloquial and not excessively formal. In due course. Perhaps also: In the fullness of time.
This locution can be used in everyday speech but as well in formal writing, to the contrary of what a former answer asserts. What is colloquial in the way of a locution constructed with "voulu" is the locution "c’est voulu", which is to be found under "A" below.
(LFi) Loc. fam. C’est voulu : C’est fait exprès, ce n’est pas un hasard.
"En temps voulu" is not unique as a locution to be constructed with "voulu"; there is a whole family of such locutions, first of all the synonymous "au moment voulu", which register is the same. In fact it is a freely constructive family. This is so because "voulu" is an adjective, a full fledged adjective, and so you can construct all sorts of noun phrases with it. Below, "B" is the relevant meaning.
(TLFi, II) A. − Qui est fait, réalisé, obtenu volontairement.
B. − Qui est commandé, requis, imposé par les circonstances.
- les conditions voulues, les ingrédients voulus, la personne voulue, le ministre voulu, etc.
Note that while there is indeed the possible meaning of "in due time" or "from a timely manner", when for example you need to submit something before a given date, this expression is also often used as a more "elegant" way of saying "later":
Je sais que nous devons encore faire cela, mais nous nous en occuperons en temps voulu.
I know we still need to do that, but we’ll take care of it later.
Was it that usage or the other one, both are rather formal from where I see it. I’d rather use it in a professional / administrative setup but could use it with my friends and family without sounding too weird.