This is not financial advice but rather an example of how I am answering my own question of when to exit GBTC. It could be very soon if we have a 'sell the news event' coinciding with larger market manipulated news.
My last entry on GBTC was around 17.50. We have been on an extended run that will come to an end at some point. That's why it is important to mark some 'lines in the sand' that will force myself to take profit, and when to completely exit.
I have been drawing out lines where I would be happy to take profit on this current run, and so far have not had to. The next 'line in the sand' seems to be appearing now. Importantly, I tell myself to not get stopped out while the market turns around the next day. So I am ok with taking profit 5% below the current price (29.25), and exiting 12% below the current price (27). In my experience when we have swings that volatile, it is better to stay on the sideline and wait for relative calm, or pick up a better swing trade if I need to take advantage.
If we continue up for the remainder of December, the next 'take profit line should be in the 31.50 range. Time will tell. Until then, I have my current lines in the sand. Hope that helps.
Happy and healthy holidays to everyone.