I've looked up the Cisco site but haven't found any reference to any time limit within which you have to give the second exam.. however don't take this as gospel since I haven't recieved any confirmation of that yet, I will look around a bit more and speak to some people.
I would recommend though, that you try and give the second exam as soon as possible. I figure you've split it into two exams since you wanted to make the studying easier.. so while you still have the first exam info in your head, take some time out, study for the second exam and give it at the earliest..
For people who want to break up the studying.. I would say give the INTRO exam.. take 1-3 months off (depending on how confident you are) and then take the second exam.