How many minutes does a 45mah battery last?

That completely depends on how much current is used by the circuit or device that's

connected to the battery. For example, if there is no circuit or device connected to

the battery, and no current is used, then the battery lasts for quite a long time.

But if you lay a screwdriver across the battery terminals and create great balls

of fire, then the charge on the battery may only last for 1/2 second.