Your backups aren’t worth much unless you make a quick test by restoring a small file or two after the backup is completed.
Actually, you can make backups at any time. However, it is easier and safer if you have a specific schedule. The first day of the month is easy to remember, so I would suggest you back up your genealogy files at least on the first day of every month, if not more often. (My computers automatically make off-site backups of all new files every few minutes.)
Given the events of the past few months with genealogy websites laying off employees and cutting back on services, you now need backup copies of everything more than ever. What happens if the company that holds your online data either goes off line or simply deletes the service where your data is held? If you have copies of everything stored either in your own computer, what happens if you have a hard drive crash or other disaster? If you have one or more recent backup copies, such a loss would be inconvenient but not a disaster.
Of course, you might want to back up more than your genealogy files. Family photographs, your checkbook register, all sorts of word processing documents, email messages, and much more need to be backed up regularly. Why not do that on the first day of each month? or even more often?