Assist timer

Hi, i really enjoy league of legends on a daily basis. But what really grind my gears is the timer for getting an assist. I dont know what the timer is. But what i do know is that when dealing 90% of someones health, that should be considered an assist even though it was a space of 10 seconds before someone else got the kill. One thing that also is a thing you must consider is assist for tanking damage. For everyone knows that someone could be the reason for a kill even though they didnt hit the victim. And in that case its mostly beacuse they are the one baiting the kill.