This all depends upon who's all in your party, and how much practise you can stand to let them get. If you look at the spell lists and what spells are really needed, and who else in your party will be able to cast them, that will tell you a lot. For Bish-to-Alch, i think you can change as soon as you get all the books from Crock, that really expands your mana pool, and leaves picks for L5 spells before you get all the way to Ferro. That's why L9 is good for me, and the only reason to go higher is if there's nobody else to cast a spell you find vital from the other Schools.
One of my fun characters was a Bishop-to-Fighter who switched at L13 total (one for Stealth) to get Portal, so all 4 chars could cast it.
Staff of Doom is quite a treat to Berzerk with.
Mostly, though, i don't switch Bishops, because i have them for the purpose of learning all the spells, and if you get that far (L22; if you switch at 18 and leave the 4 Alchemy spells, you can get all the rest) you might as well keep improving all 4 Schools, because it's too late to make up much in XP/Level ratio by the end. (unless you've got a tiny party)
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