So im trying to make onitemrightclick stun entitys in a radius and have no idea how, im using a custom tooltype and its in 1.16.4. Also is there a way to make the player have to hold right click for x time to activate it.

Its definitely possible to have an item requiring the player to hold right click for some time in order to use it. Think for example the bow, or foods. You would need to override the method getUseDuration and return the value you want. Then you would need to override the onItemUseFinish method to handle the logic of your item. As for the logic itself, if you want to stun entities in a certain radius from the player you could just use World#getEntitiesWithinAABB and do whatever you want with the Entities that you get from the method

Thanks for the reply i will give that a go