Normal time to get oath letter

The short version of my perception:

Little or nothing can be gleaned from how long it has been. Simple as that.

The odds of RQ are low, but that is because the odds of RQ are generally low anyway, and, most RQ is for obvious reasons and so is identified by at least the time one presents passport(s) at the test interview. After that point in time, however, it is my impression that one really cannot discern, one way or the other, much at all (if anything) from how much time passes.

Patience is in order, at least until the delay is inordinate and unreasonable, at which point pursuing formal channels to obtain information may be in order, including perhaps getting one's MP involved. But one really should be patient and wait.