In 6, it's determined at creation and never changes. Each school has its own regen value. Pure casters get 2-3 across the board, except for the Bishop which gets slightly less than the pure casters. (E.g. mages get 3 in fire, water, and divine and 2 in the others; priests get 3 in earth, air, and divine and 2 in the others.) Nonmagic classes and hybrids get 1 across the board. 16 vitality at creation gets you +1 to all schools. Faeries also get +1 to all schools.
In 7, it's the same basic system, separate school regen values fixed at creation and never changing, but the numbers are different. Nonmagic and caster class values are mostly unchanged, but hybrids get 1-2 depending on school instead of just 1. In 7, both piety and vitality affect the regen at creation, with piety having double the effect of vitality. For example if a Mage wanted a +1 bonus he could have 16 piety and 10 vitality, or 15 piety and 12 vitality, and so on. It's theoretically possible in 7 to get a +2 from stats, but you either need 18 piety and 15 vitality, or 17+ in each. Bishops now have full caster regen instead of a penalty. Faeries still get +1. (Max regen for a faerie bishop would be 6 in fire/water/divine and 5 in earth/air/mental, but would require rolling a 25 at chargen for the stats, which is less than 1% odds normally.)
When importing 6->7, your mana regen gets recalculated, but your stats also get a slight nerf, but in all likelihood you'll get better regen in 7 than in 6. The game doesn't remember your previous classes, only the max level you've reached before class switching, so if you switch a priest to valkyrie in 6, it'll import using valkyrie regen values.
Regen in 8 is based on your piety, and also the number of spells you've learned for a given school. However, there are a large number of mana regen granting items in 8, which generally have a much larger effect on your regen than anything else. Being a faerie is a more noticeable increase than having 100 piety is, and even +1 mana regen from an item is significantly stronger than the faerie bonus. When you import 7->8, however, your stats are AT MOST 15 points higher than usual (out of a 1-100 scale, across 7 stats) so the benefit from importing is negligible. Importing is still useful because your combat and utility skills will be much higher than a starting party and you can bring some very useful midgame items along, but importing provides only marginal benefits for spellcasters.
TL;DR: It's very very helpful to start hybrid classes in 6 as their pure caster variant and then swap later, but it doesn't matter nearly as much in the other games, especially when you're importing. Class changing is extremely strong in both 6 and 7, but you're usually crippling yourself if you do it in 8, so make sure to finish all your switching before finishing 7.