Depends partly on your password. There are commercial crackers
available that can crack dictionary word passwords in minutes to
hours. Random character (including punctuation) passwords can take
hours to weeks, depending on the horsepower of the machine(s) used.
You need to analyze who's likely to get at it. If you don't have
anything supersecret, and your machine is a desktop under your
direct control, a random password is probably adequate. A laptop is
a little less secure, but most laptop thefts are wiped for resale,
not mined for data recovery, so you may want to take the chance.
(Of course, if you have kids, you're toast.)
Note however, that password protection prevents opening the workbook
in XL. It doesn't encrypt the file, so using a hex editor can glean
all the text, and many of the numbers from your sheets.
I personally keep all my files on an encrypted disk, and use PGP to
strongly encrypt and password protect my financial and other
sensitive data. But I'm a bit paranoid.
