It of course depends on how you read it heh. It took me about 2.5 months of 6-7 hours a day average to read bio, ochem and genchem (of course, with doing problems and so on). Got TS 24 on the real DAT, mostly because I reread the textbooks.
PS: I took gen chem in 04-05 and Ochem, Bio in 05-06 (more than 3 years ago).