8 hours will unlock every character in SE except for 3 which are....

You get the first and 3rd by going back in SE to certain levels and fighting them.
The second is unlocked by
You are looking at up to somewhere near 450 brawls to get all the characters, which could take way over 8 hours.

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{Spur of the moment Toad Mario Kart poem below! lol it is bad!}