As a physio who treats gynae patients, can I please advise not to do regular mid-stream stopping, as this can cause problems with back flow. if you want to assess every now and then as a test fine, but should not be a regular thing.
As for frequency of exercises, needs to be divided into sustained holds and short hard squeezes. Pelvic floor muscles are divided like most muscles into fast twitch and slow twitch fibres. the long holds will work on slow twitch fibres (marathon training) for everyday activity, short holds will help fast twitch fibres for when a short direct force is put on pelvic floor (i.e. coughing, sneezing etc).