in general, I would say around a minute (at that point your interviewer will start to become a bit unpatient). However, this does differ across consultancies and interviewers.
Most of the time it does not get awkward as long as you keep doing something. It only gets awkward, once you only stare on that piece of paper for too long, so try to avoid it.
Just start outlining your structure by saying: "I would just take a bit of time to structure my thoughts, if that is okay" (never go for "I would take a minute" - I have seen interviewers who will interrupt you exactly after one minute...)
It may happen that your interviewer will interrupt you nevertheless. This should not stress you out, just stay calm and finish your structure "live" together with the interviewer.
For preperations, I suggest you keep an eye on the time it takes you to outline the structure and, if it is more than one minute, try to speed up the process a bit.
Hope this helps!