in my company they give you a training record which lists all the topics you should know and will be quizzed about during your line training. There is a supplement there that lists all the relevant chapters in OM-A/B/C/D that one can cover to prepare. Basically in order to be released for line check you have to have all topics marked with sufficient grades from your line training captains. Generally the way it works is - you take off, establish a cruise and if the workload is low, the cpt simply asks which topics have you prepared and are ready to discuss. So basically you prepare at your own pace
But the theory questions really don't begin until you are some 10 sectors in and somewhat comfortable with whats going on. Until then really don't fall behind the plane really and don't plant it into the ground