A good while ago someone did a very nice comprehensive chart. What it showed was that AP provides the best burst up to around 200-250 AP. However after that you get more burst from adding AD rather than going to 300 AP. This isn't just sustained damage, this is her actual burst; the more AP she gets, the more effective AD becomes. For instance, buying a BF sword at 300 AP added a significant amount more damage to her combo than going to 400 AP did.
Akali does well with AP items but if a game goes long you should add AD rather than super pumping AP.
However does this take Lich's Bane into account?