Is it equal to their levels in the prestige class? To their character level? To their level -3 like a paladin?
It doesn't appear to be spelled out in the prestige class.
I believe it's equal to their PrC level but the text doesn't spell that out :(
The Caster Level section of the Magic chapter has this to say about caster levels:
Caster Level
A spell's power often depends on its caster level, which for most spellcasting characters is equal to her class level in the class she's using to cast the spell.
Since there's no specific rule overriding this general one (like there is for the Paladin) and they aren't building off of pre-established caster levels, I'd assume equal to their levels in the prestige class.
That's the quote I was thinking of...but my phone hates me and will NOT load the pfsrd and won't copy off the paizo site :(