How long it takes for GRE score to be delivered to graduate school?
Per the ETS website, for the online general GRE (emphasis mine):


  About 10–15 days after your test date, your official scores will be available in your My GRE Account, and your scores will be sent to the score recipients you designate


It is up to the receiving institution to decide how they want to receive those scores: online (which doesn't add additional delivery time), or paper mail (which adds some additional shipping time).