Check your Design Rush tracker! You'll see a timer on the node where you earned the reward, indicating the amount of time left on your boost. You'll also see the Design Value animation increase twice after you submit a challenge, showing that the boost is active.

And be sure to keep playing! The time will count down even when you're not actively playing.

As a courtesy, you will receive a five-minute and one-minute warning when your boost is almost up while designing a room.