I have recently given both blood and platelettes.  At NYU Medical Hospital in NYC the told me this:

You should wait at least 2 weeks before donating platelettes again.  (or to donate blood after donating platelettes.)

You should wait at least 6-8 weeks after donating whole blood before you donate whole blood again or donate platelettes.

Isn't it nice to help people?!  Congrats on giving blood (and platelettes)!  Did you know that only 1 person can use each whole blood donation, but 5 people can use one platelette donation?  And after donating platelettes, you dont feel dizzy or weak, and you can do excercise, play with your kids, whatever!  After donating blood you should rest and take it easy for the rest of the day.  Either way, you are saving a life!  Thanks!