In a normal game, respec will be available around level 10 or 11. Some skillbook like barage, eye gouge, whirlwing are available only once. If you respec after learning these skill, you can never learn it back again.
It's not tied to level. It's tied to the number of star stones/blood stones you have consumed.
I'm almost level 14 and haven't enabled respec yet, even though i can (i'm holding some blood stones because i don't want to be without some if another quest like Alfie's requires one)
It's not tied to level but I provided a general level indication for typical playthrough. It's important because sometimes you may want to reroll if the respec is not available or not practical.
