Just got one. Can't speak of how often it need to be calibrated, but distilled water and less than 2 minutes to do it. There is a scale in the view finder and it'll tell you what you need to know - exactly where you are. 2 or 3 drops of your tank water on the glass, close it and look towards the light (or spend more to get built in light). Takes no time at all.
Nice tool to have. Beats dipping the swing arm or floating hydrometers.
"Life may not be the party we hoped for but while we are here we might as well dance!"