When can you deliver the manuscript? How many months will you need to complete it?
Give them your best estimate. And be sure to include time for when your life blows up and everything in the universe keeps you from writing.
In other words, make a schedule you can live with. Then tell them when you could have your manuscript done if they asked for it.
A good amount of time is usually six months, so you could say:
Manuscript Delivery: I could deliver the manuscript within six months of a signed contract.