If you take one 60 mg dose every 12 hours, you are getting on average 60 mg/12 hour = 5 mg of
morphine per hour. If
you taking 120 mg over a 12 hour period,
you are getting on average 120 mg/12 hour = 10 mg of morphine per hour.
Take 25% of those morphine mg per hour numbers above and that will yield the bioavailability of morphine (i.e.,how much of the dose in the pill is actually active towards relieving your pain).