If your project is time sensitive this should be one of the first questions to ask. Remember you're trying to weed out every candidate who isn't a good fit. If you know you need a website done by next month then be clear about that.
And if the prospect answers this with, "Maybe" ask them to please tell you, "Yes" or "No." Maybe doesn't cut it. And you don't want to maybe count on this guy to screw up your project. It's pretty simple… "'Yes I can meet the deadline" or "No, I can't."
Also remember to be clear about time of day you expect it delivered and which time zone you're in. If your freelancer is in Portland, OR and you live in London that's an eight hour difference. So end of business day Thursday in London would be 5:00am in Portland.
A side note here: the good freelancers are typically booked out at least one month. So if you're looking for quality work I'd giving yourself plenty of time to find a good freelancer. The freelancer who has nothing scheduled for the next week makes me wonder, "Why not?"