I thought, I would want to know a couple of things: my location and my activity for every second of my life. Since I generally live relative to the surface of the Earth, my location could be stored in a GPS coord. And since the number of activities I engage in are not that big, I could probably store them as indexes into a map and a 32 bit int would probably give me a sufficient number of unique activities. I doubt I would exhaust that index space no matter how many unique activities I engaged in.
The math is pretty simple from there. Number of seconds in your expected lifetime multiplied by the size of data you want to store per second. Also consider how much of that is compressible (like an ideal 8 hours per day of sleep). Pretty sobering when I think about it.