I don't believe the option to vary the frequency of backups exists within Time Machine: it's either on or off, and it backs up hourly when it's on. See for example
OS X Mountain Lion: Time Machine preferences.
Beyond that, though, I don't know why the backup frequency should correlate with intensity of computer use. Time Machine only backs up
changed files, so if Rita's not using her Mac much, Time Machine will wait patiently until she does.
