If you only use it for like a few seconds (outro) then you don't need to ask for permissions because it falls into Fair Use. There is lots of info about it but yeah
If you had read any of the information regarding fair use, you would know that this does not fit the requirements.
It only being a couple of seconds does not make it alright to use.[DOUBLEPOST=1468239140,1468238651][/DOUBLEPOST]
I have made over 40 videos +/- with that outro and none of them where copyright claimed, but only this one video?
The Outro was 15 seconds and I credit the owner of the song in the description
All that tells me is that you have 39 other videos which could, at some point, be claimed and monetized by the song owner.
Crediting the song owner is meaningless and does not give you the right to use it without permission...nor are you safe because you only used 15 seconds.
Without the proper permissions or fulfilling the requirements for fair use (you don't), there is no safe amount of music you can use. Any use- need it be 1 second or the entire song is unauthorized and you could expect to be monetized or restricted in certain countries.