If most are like me, it is always a work in progress. As new things come out or features get added, I try to implement them. My sig is not updated. I have recently added XM and did the Lilliput indash conversion. It is a great hobby and if I ever get finished with it my wife will start finding other things for me to work on.
Back to your question though. Assuming all parts that you want to implement are on hand already, you could probably have a working system, bug free for the most part in a couple of days. It all really depends what you would like to have into your system. The more you add, the more things you have to make work together the way that you want them to.