General rules for blood donation:

Age: Donor must be at least 17 years old. 
Weight: Donor must weigh at least 110 pounds. 
Health: Donor must be in good health and feeling well on the day of donation. 
Travel: Donor has not been out of the country for the last 3 months.

Second Blood Donation : The time duration for the second blood donation after donating blood for the first time is 56 days.