A 'poster study', or a quick, small-scale mass sketch is helpful to solve issues in getting the visual impression from a particular composition, scene, pose, etc. It's meant to be the broadest and simplest possible expression of what you see, but also must contain some basic information that you could use in aid of the actual work itself.
Time taken ideally for a poster study or sketch? Depends, really on what you want to achieve. Most of these were done in various drop-in evening classes, sketching sessions, life drawing sessions, from photographs, etc etc etc. usually when working in mass is more preferable than linework. They usually stop short at the 'block-in' stage, right before attempting to correct/amend drawing and considering edges. It's quite fun doing poster studies, and a goal once set by a tutor was to attempt 50 successful poster studies in the course of a single year to force yourself to think in much broader, simpler terms in mass expression.
Procedure was like zapping solid blocks of mass in place until the overall impression is reached at, or the entirety of the support is covered. The compromise between accuracy of drawing and the overall vision as defined by the restrictive time limit is something that has to be learnt as well- the broader the overall vision, generally the more accomplished the draftsmanship involved.