It depends how many exam you have, I have 5 and started last wednesday and have already nearly completed the specification for F212, got F322 and G482 to go and hopefully it will take me two weeks to cover everything I need to know about my exams. With that being said I'll only have about 2 weeks until my Mechanics 1 Exam so hopefully that's enough time. Till the science exams I will be completing past papers.
I know people with 9 exams (retakes) that started straight after the January exams.
It depends on your revision technique, and how long it will take you to cover and know your topics. If you're resitting them all I presume you have 9 exams, for which I'd say, yes you have left it a bit late but you could always concentrate on the unit 2's more as they are worth more.