I bought my 3DS on July 27th, 2012. There have been about 576 hours since then.
Of those 576 hours,
about 144 were spent sleeping
about 102 were spent at work
about 22 were spent driving to and from work
for a total of 268 non-discretionary hours, leaving 308 discretionary hours.
According to my Activity Log, I have used my 3DS for a total of about 97 hours to date.
This accounts for approximately 31.5% of all discretionary hours since purchase (though this does include considerable amounts of multitasking, as well as the ~2.5 hours to transfer my DSiWare games and photos).
Nearly one-tenth of that time was spent playing Mario's Picross, which I just bought two days ago.
(Despite the biological imperative, I do not count eating as non-discretionary hours, as it would be possible to watch Netflix on my 3DS while eating.)