For Cialis, the published "daily dose" is 5mg. I can only get 8 pills/month regardless of type/strength, so I buy 20mg and cut them into more or less quarters.
For Levitra or Viagra, I don't have a "daily dose", but Dr. Mulhall's book (Saving you sex life, 2010) p.102 says for those who show good response, rehab-focused options are:
Viagra 100mg or Levitra 20mg 2-3 times a week
Viagra 25mg or Levitra 10mg 4-5 nights/week
Cialis 20mg 3 times a week
I was told that I could also cut Viagra or Levitra pills, so buying larger and cutting them up is an option there too.
I take the Cialis 5mg +/- daily.