I recommend once every 3-6 months at the most. Day-to-day and week-to-week fluctuations are as meaningless as turn 5 of a Civilization game.
Actually if you look at bitcoin as an investment that's not wise... it feels good to say it and lots of people love to say it about their 'investments' but it's simply emotional pandering, not absolute truth. Lots of people continue to hold losing stocks as they crash down to zero, being emotionally stubborn when a simple stop loss would have saved them tons... not saying that's the case here, I'm just applying this in a broad financial sense. Saying it's not a loss until you 'sell' that it's just on paper is another thing said by people losing money. Winners know better. Also keeping to the position that bitcoin is a currency thus no need to convert it to USDs again is a very noble philosophy. We shall see how it plays out.