Is 60mg morphine a lot

For an opiate-naive patient, 60mg would be a lot. The weight of the patient should also be taken into account.

So, say there is a patient who weighs 180lbs (~90kg) with a broken leg, I would start them off on 10--15mg of morphine via an intramuscular injection.

After 15--20 minutes, if the patient was still in excessive pain, I might give another 10mg. I'd repeat this, if necessary, up to maybe 50mg, or possibly 60mg.

One would NOT start out giving 60mg. It may not kill your patient outright, but it could cause excessive sedation and respiratory depression.