S.B.
If you're talking about the Pedia-Lax suppositories, my pediatrician recommended them when my son was just over a year old. You can use the full "dose," and for my son, it worked within minutes of application. Now, the process of administration wasn't a lot of fun for any of us, but he was so much more comfortable after it worked its magic.
If you're talking about an oral laxative, then I don't know the dosage. I'd suggest trying the suppository if you want quick results.