Si no vuelve, en unos días ya podríamos operar sobre [la pregunta].
I'm looking for a rule of thumb, a number of days to wait.
It is nice you ask this in Meta because 1) I forgot to answer to your comment in the linked question, and 2) immediately after writing that comment I asked myself the same question :)
To 1) I am sorry, these are busy times.
To 2) I agree with walen that one week can be some reasonable amount of time: it gives enough time for the OP to come back and not enough for us to remember that question exists. However, to me the underlying idea here is what mdewey explains in this related question of yours:
Leave a gentle comment and then wait. If the OP does not come back to clarify the post and narrow down the questions within a few days then the question should be flagged for closure as too broad.
That is: we are here to solve problems. If someone enters this site and has a question, it is his responsibility to express it in a clear way. We can help them to express it better, improve its format and many other things, but the core of the question is still theirs.
If they happen to just vanish, we have some options:
As they say, Spanish Language Stack Exchange is moderated by you. We together can build a better place in the internet. It is our choice to improve things: if you do, tons of people (+10K every day!) will silently thank you. If you don't, please do not feel bad. So neither feel the urge to ask something the OP did not really ask, nor to clarify what others do not make clear.
As I once read in Stack Overflow:
Don't polish turds
The purpose of editing is to make a post easier to understand, and easier to find. If a post should be deleted, then flag to close/delete instead. Editing these posts is sometimes called "turd polishing" - no matter how much you polish a turd, it'll always remain a turd. Similarly, if a post is inherently worthless, it'll always remain worthless, no matter how much you edit it.
Instead, try to find hidden gems.
As a rule of thumb, I would say a week is just enough time.
It's long enough to allow for people that enjoy offline time on weekends to come back on Monday, as well as people with a busy week that can only post in the weekend. Etcetera.
It's also short enough for mods and seasoned users to not forget about it and edit / enhance / split / delete it once this waiting time has passed.
If somebody made a question and then ignored answers and comments for a whole week, I'd say they weren't that interested to begin with.
I've been going back and forth between two days and three days, i.e. 48 hours and 72 hours. After all, if OP doesn't like the revision, he can always roll it back.
One of the reasons I like to try to rescue questions that are about to get closed, or have been closed, is to encourage a new user to USE the site. If their question doesn't get answered at all, he's liable to get discouraged and not come back. (I realize there's still a good chance of that happening even if I rescue his question, but I still like to try. Sometimes it turns out well. I've had some success with this at ELU.)
I'm not going to try to convince anyone else to go around rescuing questions. But I'd like to be able to rescue some questions here with a bit less interference than I've gotten. That's my motivation for asking this question.
A week seems ridiculously long to me.
The only reason I was thinking of two or three days was that I was advised to wait a couple of days before rescuing the question. Frankly I really disagree with that. When I write a question on any SE site I want as quick and helpful an answer as I can possibly get because 90% of the time I have something on my mind that I want to figure out ASAP. (The other 10% is intellectual curiosity and there's no hurry for that.)
(A question such as the one I posed here at Meta is completely different. With this question there's no hurry whatsoever.)
If we fix a new participant's first flawed question or even the first two flawed questions, then after that we can fade away and make it more sink or swim. That's my view.
I would take this general approach on any SE site but ESPECIALLY Spanish where we need to stimulate more growth.