@bluesami -- unfortunately there is very little available information about the timeline for applications subject to RQ-related (Residence Questionnaire related) non-routine processing.
As
@rajkamalmohanram noted, and you appear to be aware, the requests for additional documents are what we generally refer to as "RQ" or "RQ-related," meaning they are requests for additional information and documents regarding proof of meeting the actual physical presence requirement for a grant of citizenship.
Who and why, are each separate, complicated subjects, and frankly off-topic in this thread.
When is currently, it appears, usually at or following the test and interview event. This is clouded more than a little, currently, given the extent to which Covid-19 has skewered things. Leading to . . .
What is the impact on the processing timeline, or how much longer will it take because of RQ, is less complicated because we really do not know. Cannot say much about it beyond it "DEPENDS," and that is a partial answer at most. But, currently, given the extent to which Covid-19 has skewered things, RQ-related non-routine processing does not appear to be seeing much if any progress, so waiting and continuing to wait, with potentially some rather long processing timelines looming, seems likely.
@rajkamalmohanram -- RQ-related non-routine processing has been, probably, the number one focus of my interests here for more than a decade now. I appreciate any and all efforts to gather, digest, collate, and analyze information about this aspect of processing and deciding grant citizenship applications. If you at least copy or comment about what you learn in the topic titled: RQ versus Physical Presence Questionnaires, including CIT 0205 here:
https://www.canadavisa.com/canada-immigration-discussion-board/threads/rq-versus-physical-presence-questionnaires-including-cit-0205.534082/ that might help to keep access to a somewhat centralized repository of such information.
I will quote your list there and attempt to address it later (been up all night to get a day-job manuscript submitted way past its deadline, so what's left of my brain is slip-sliding away for the day).
For now it warrants noting that ANYONE, ANY APPLICANT, can get RQ-related requests. IRCC does not need a "reason" to make such requests. In the topic I referenced, for example, one version was clearly being issued randomly as a quality control exercise.
In that topic, and in many others, I have often discussed factors which increase the risk of RQ-related non-routine processing, to a significant extent based on legacy information about what were known
reasons-to-question-residency, called triage criteria in the File Requirements Checklist introduced during the Harper years. The criteria has been confidential now for nearly a decade.
But for any applicant who has received RQ-related requests, the *why* does not matter all that much. Likewise, really, for anyone who has already submitted an application. Discussions about what triggers RQ is more about recognizing what can increase the risk of RQ, and thus is mostly of interest and use for PROSPECTIVE applicants, PRs still considering when to make the application.
There is no way to avoid RQ. There are ways to minimize increasing the risk. Just getting the travel history complete and accurate, for example, is among the biggest; applying with a decent margin over the minimum, probably a big one. Following the instructions and making few mistakes overall, likewise.
In the past, certain factors like self-employment were big factors (for a time in 2012 ANY period of self-employment triggered RQ, as did any period of unemployment). Probably still a factor, but its weight is largely unknown.
I have already wandered enough into the weeds here . . . but, it demands a reminder: for most of the relevant factors, it is not so much about a particular factor in itself, but far more about how various circumstances relate to one another. Note, for example, if the applicant is a U.S. citizen who reports zero travel abroad, that's going to trigger some questions; in contrast, for a PR in Canada as refugee, no travel abroad is not likely to invite concerns; for a factory worker, holidays abroad for months at a time probably invites some probing, but for a university professor not so much. It gets complicated.