We have all faced with the situation where there is too much to do an not enough time to do it. Sometimes the technique of Time Boxing can save the day.
Time Boxing is useful when you have tasks like documentation, specifications or other analysis type work.
Say we have a system that need analysis performed. You estimate it will take 200 hours to complete the task. Well, what if you only have 75 hours.
The trick is to cover the width of the problem domain without going into the depth completely. So, as the diagram on the right depicts, you cover the problem domain an inch deep and a mile wide instead of covering the whole topic. The trick is cover enough width to have a meaningful deliverable.
It takes a bit of practice, but over time you will gain experience with this approach.