In my experience normally about 3-4h.
But its possible faster. Once just after minutes I had a "Gold has been sent" mail. And an hour later I had the gold in my ingame mail (You normally get two mails: one to tell you that your order has been placed and they are processing it. And a second one witch says they have send you the Gold and you should check the amount and tell them if you have received it.) the longest it ever took was once 7h.