Black Rod was interviewed earlier and he said that they had done many rehearsals over the years and worked out the timings for getting people through Westminster Hall (by using actual people to test how quickly it can be done). Having to stop the lying-in-state so the actual funeral can take place is going to be the limiter on how many people can pay their respects rather than how long the queue is. Which is why there is talk of if/when the queue gets to Southwark Park, they will have to start turning people away as they won't make it to Westminster Hall in time to be allowed in.
He said that despite his position, he will also need to join the end of the queue (and will be doing so).
He also said that they are asking people not to stop at the coffin and bow etc as that slows down the movement of people and means that less people will be able to see the Queen. I assume that this may be why they didn't do timed tickets as they want to try and let as many people get in as possible.