Self-answer: Enter just the portion related to those actual months. TurboTax won't actually divide (so I'm not sure why it asks the question in the beginning).
I figured it out because I just now noticed hyperlinks at the bottom of these pages, which say:
"How do I enter my mortgage interest if the time I used the office and the time I paid mortgage interest are not the same?"
"Didn't have the office for all the months I paid expenses in this home?"
For example, the answer to the second question says the following:
"Since the time you lived in your home and the time you had your home office are not the same, enter expenses only for the time you used the space as a home office, not for the entire time you lived there.
For example, if you lived in your home all year but didn't have a qualified home office until August 1, and then had the office the rest of the year, enter expenses you had from August 1 to December 31.
Your expenses for the time you didn't have the office are considered personal expenses and are not deductible."
