The "sit tight" is until you reach the goals you set out before you invested. When I started investing in silver when it was under $5, I set a goal of 4x's my initial investment. What ended up happening was just as I was about to sell @$21 - the market crashed back to around $7. It was several years until it reached that price again.
One of the problems with cashing out Bitcoin and buying a house is taxes. To buy a house, you will have to convert Bitcoin into Dollars and enter the banking system and government reporting. What is the Capital gains tax on the profits? That could be a major deterrent for those with large gains.