Well I can talk about Empathy. What do you mean by...
spells? Empathy isn't "cast" as such. It is a constant arcane ability with no real way to turn it off. Consider it much like you would mainstream telepathy in... X-Men or something. It works in much the same way in terms of constant use. As such, yes, at some point, an empath will be weakened/exhausted by use of the arcana but there is no true hard and fast limit on how many times you can 'use' it because it's always being used. It would really come down to common sense and good judgement on the part of the player. No empath can last forever in active use. They will be overwhelmed by the emotions long before they are tired out probably. But passive use is just that: passive and constant.
Like I said, consider X-Men: characters in the films can have massive surges of power for active use for a brief period in the
deus ex machina moment of the film's ending and then they sort of... flop and fall unconscious or whatever. But the telepaths always have some low-level buzzing in their ears.. you know? Empathy is like that.
