Time Machine will simply back up all changes since the last backup, each & every time it can see the drive.
It runs in permanent incremental mode, with auto-clean-up of oldest data when the drive gets full.
It will not save hourly versions of documents, if it doesn't see them every hour.
An alternative method might be to use an internal drive/partition as a 'quick recovery' Time Machine & clone that to your external once a month. Time Machine drives can be simply file-copied.
Or... 3rd option, use a small internal Time Machine [I'm presuming you are using the external for space saving reasons] which will not be able to keep data for long, then once a month, set the external to be the new Time Machine, reverting to the internal once the external is finished backing up & unplugged.