Unbalanced champion? or balanced champion?

So i have long held that my champion of choice is bad because they take "too long" to be viable. So i figured i would ask this question without naming any specific champions.

So first to those who answer, what would you say your average game length is? i know mine is probably about 30 mins, unless a team surrenders. Once you have that number in mind answer me this is, "what percentage of that time does a champion need to be viable to be balanced"? By viable i am talking that the champion has a notable effect on the game that is not limited to landing their ultimate or some other minute+ long cooldown. This can be an engage / disengage tool, the ability to reasonably kill an enemy champion, the ability to keep an ally alive, etc... Champions can start viable and become not, like an early power champion who does little to nothing in late game team fights.

My champion of choice has an issue where i would argue they aren't viable for about 60%+ or more of the game. The running opinion seems to be that since their late game is abnormally good, it's not a problem if they spend that much of the game under whelming. So as a follow up, would anyone play a champion with a very strong early game BUT one that falls off really hard in the mid to late game (to the point where they are "useless" after the 15-20 min mark).