according to the answer here http://wiki.answers.com/Q/How_many_times_does_the_average_person_blink_in_a_year
"It is estimated that people blink 22 times a minute. Thus, it can be calculated that people blink 960 times in one hour; 15,360 times in a 16-hour day "
so we have 15360x365 = 5,606,400 per year, then 50 billions divided by 5,606,400, we have 8918.37899543379 years
George, with no other replies, I'm taking it that everyone else in the community is trusting your calculations - but, the Facebook fans are disagreeing with you. Compare your math with them...ARM - Timeline Photos | Facebook
Wikipedia states that "actual rates vary by individual averaging around 10 blinks per minute in a laboratory setting". This gives one blink every 6 seconds, so 50 billion blinks = 300 billion seconds = 3x10^11 seconds. One year is about 3x10^7 seconds, so a simple division gives you a nice round answer of 10,000 years.
[Edit: I forgot that we sleep. Bump that answer up to 15,000 years.]