Terminal length is simply the length at which your hair will not get any longer. This is basically the time that it takes your longest hairs take to grow all the way out from the beginning. It could be 5 years or so for this to happen. Every single hair will not be at terminal length at the same time. Not only will genetics determine how long the hairs will be but premature shedding, brush damage, and split ends will limit the length as well. I have heard numbers of head hair terminal length as low as 12" and beard terminal length as low as 6". Member Fuzzy here has gone 6+ years without trimming and is pretty much at terminal length around 3 feet. Most people will never see their true terminal length though.