Personally I started hitting the question banks about a month prior to the exams, making notes of the ones I got wrong and deciding if it was a stupid mistake or whether I really didn't know that subject well enough. If it were the latter I'd re-read the relevant chapters.
And repeat... until I was getting pretty much 100% in the question banks.
By the time you show up for groundschool you should be ready for the exams, it's just to clear up any principles you can't get your head round (let's use lambert charts and gyros as common examples!).