Can I specify the sync frequency on Google Drive on Windows 8.1?

This does not seem possible. The approved answer on this post on the Google product forums specifically states "Drive syncs automatically when you add/delete files from the Drive folder on your computer."

There is a program called @MAX SyncUp that states it can "copy data on schedule or only when files are changed". It is free for personal non-commercial use, but the free version has the restriction that you can only set up one synchronization or backup profile.