I take two readings, one when I pitch my yeast and one before I bottle. I know that the beer is fully fermented after 2 weeks, so I just do it to record my FG. I used to take a reading once a day for the last 3 days, but I have decided that it is pointless, the beer is always done after two weeks. Now I will be letting my beer sit in the primary for 3-4 weeks, so there will be no question if it's done. I always just put my Sanitized Hydrometer right into the fermenter. Never had an issue.