From the link http://studentaffairs.stanford.edu/gradadmissions/admitted/offer, it states this:
"Accepting or Declining the Offer
Once you complete the SUNet ID application process, it will be approximately 24 hours before your SUNet ID will be available. You can then log into Axess to accept or decline Stanford’s offer of admission.Note: If accepting, it will take an additional 24 hours before your access to other Stanford online services, including email, will be available."
So, you should be able to access it immediately after creating your SUNet ID, so that you can respond to their offer of admission.