For you (as well as your referral) to get credited, your referral has to buy at least $100 worth of bitcoin (cumulative sum) within 6 months of signing up.
If your friend visits coinbase.com/trade and initiates a buy of 100 USD or more (or 100 USD equivalent of your domestic currency) within 180 days of opening his or her account, you both will receive a 10 USD (or 10 USD equivalent of your domestic currency) referral bonus when the buy completes. Buys can take up to 4 business days to complete.