A trainer, if the servos and linkages are all set up properly, should use around 70-80mah per flight. So you should be able to get 5 flights out of it before needing to charge. But do understand that caveat of everything being setup and working correctly. A failing servo or a binding linkage can easily double that current usage, so there's no substitute for checking your battery voltage between flights and doing a cycle test after maybe 3 flights to see what your actual usage is. BTW, using a smart charger and reading the mah that the screen says is not a reliable indicator. There is always some efficiency loss when charging, so that number will only be a rough approximation. Discharging the battery down will give you a good number to work with. Subtract that from the discharge test you did before installing the battery to calculate an average current use per flight. My rule of thumb is to go to half of the pack's capacity. That leaves a decent safety margin in case you miscount or do start having a servo issue.