The most natural way to ask would be
What is the probability that neither house is completed after seven weeks.
I'm not very clear, but the sentence can be used to ask about greater than 7 weeks.
If the probability that both houses are completed after seven weeks is 25%, then the probability that at least one house will take greater than 7 weeks to complete is 75%.
In the context, I assume that the houses will eventually be finished. If this assumption is not correct you need to be explicit:
What is the probability that both houses will be completed, but it will take longer than seven weeks.
What is the probability that one (or both) of the houses will never be completed, even after seven weeks?
Or more casually:
Are you ever going to finish building my house!?