There are interstitials on game over In arcade mode, or after 3 matches in career mode. You can also watch and advert on the character select screen to unlock a character.
You may not have seen an advert as I try not to show adverts to new users still trying to work out the controls. So if you score less than 3 in arcade mode, no matter how many games you play you won't see an advert. Same for career mode, if you play and lose your first 3 games you won't see an advert.
When an advert is showed there is also a button to pay to remove the adverts, only on iOS.