What is it the Damage pre-minute or is it hours?

Not long ago i was watching my rail gun beat up some tanks on 2 hp. my rail guns do 3.5 damage and there are two of them (7 in total damage). but i could not tell for the life of me how long it would take to knock them out. and if you look they fire a round every couple of seconds. so what is the DPM of the tanks ships or artillery. the only one i now for sure is planes were they do all their damage in one blow. how does this work?