You can make a purchase whenever you would like along your savings journey for the full amount saved or a lesser amount. Rewards are applied at the time of purchase on a prorated basis
For example: if you have a $500 balance including $400 in your deposits and $100 in earned rewards and you make a purchase for $250 (50% of total savings goal balance) You would use $200 in your deposits and $50 in rewards. The remaining balance would stay in your savings until you make another purchase or stop your savings.