Usually it caps at about ten casts of each. In my last game I was aiming to overkill every single enemy(harecord levelling) so I abused cheer and focus. I didn't notice that they worked any better after using then 10 times or so. Up until that point, they provided noticeable differences (like the difference between a kill and an overkill)

Cheer is also particularly useful when fighting Kottos in the monster arena at low levels: he has incredible strength, and when you have low levels, it could mean the difference between auron dying, and auron with the masamune, protected, and sentineling having 150 hp left and counterattacking :]