Well. Lets say its a non-leap year.

365 - 104 (52 saturdays and 52 sundays) = 261 weekdays

261 - 10 (assuming 2 weeks paid vacation) = 251

251 - 11 (federal holidays in 2004) = 240
http://www.opm.gov/fedhol/2004.asp

240 - 5 (a reasonable number of sick days and personal days) = 235

235 (days of work in a year) * 8 (the minimum number of working hours per day for a full-time job) = 1880 hours of work per year

365 (days in 2004) * 24 (hours per day) = 8760 hours per year you could potentially be busting your ass for the man

1880/8760 = 0.21461

or roughly 21.5% of the year a full-time employee should be working.

I am an employer, and if I had a full-time employee who clocked in for 4 hours a day I'd send them packing by the third day.