I agree with @Ingo2711 in that you might consider replacing the internal drive of that MacBook.
Replacement is pretty simple and straightforward and inexpensive, and a HDD replacement will cost a little less than an SSD. And a Time Machine backup is essential if you value your data.
Find resources here: OWC internal HDDs & SSDs - otherworldcomputing
To answer your question directly: leaving it on likely will have little impact on the longevity of the drive at this point. If it's the original drive that the Mac shipped with, it is already overdue for failure being 16 years old!
You should also seriously consider upgrading to a newer Mac and perhaps a newer version of Quicken, a program that I gave up on myself many years ago.