This is not a complicated question like other agents make it out to be. You asked a simple question and you want a simple answer, right?
Commissions are determined when the Listing Agent gets the listing. Depending on the price of the home, the usual starting point for commission is 6%. That gets split between the Sellers Agent and the Buyers Agent (3% each). However, sometimes at the time of listing the home, the seller will negotiate the commission with the agent.
All listings are different but usually stay around the 5-6% range.