Try helper text below, so there's no memory tax once they start entering a format.
Another option you can also look at is using a forgiving format pattern
This means if a user enters '2 weeks, 1 day' or '10 days' that is still recognized by the application. It's conforming the application to a human, rather than forcing the human to think in terms of data input for a machine.
From designing interfaces:
The user just wants to get something done, not think about "correct" formats and complex UIs. Computers are good at figuring out how to handle input of different types (up to a point, anyway). It's a perfect match: let the user type whatever he needs, and if it's reasonable, make the software do the right thing with it.
This might take more investment, and is not worth it at the moment, but it's an aspirational goal for UX at any time.