For the fun of it here is some math (take it with a grain of salt):
13.33 encounters per level, 4 encounters per day, Wizard 1 (3 first level spells), cast only [evil] spells.
Adventure 4 days (cast 12 spells) and then attain a new level Wizard 2 (4 first level spells).
Adventure 3 days (cast 12 spells) and get a new alignment.
Amount of [evil] spells needed = 24, when starting from level 1, to get a brand new eviller alignment.
So gradually the character changes its aligment with its own actions.