Slayer deeds do not have a cap. You can finish a deed in a day if you wanted.But they can be kinda grindy, especially in Angmar.

For PvE slayer deeds advance in fellowships or solo but not in raids. Usually there are 2 tiers of slayer deeds, the first completion is a title and the next tier is usually a virtue or a item xp rune for end game. Tier two is also twice the number of kills you need for completion from the first tier. For example if one is killing wolves in the shire, the first tier is 30 kills, the title you'll receive is fur-cutter, and the second tier is 60 kills earning a rank in Discipline.

For monster play you simply must land the killing blow the advance the deed. Each tier is significantly greater then the previous.