I just moved from OH to CA. I was (and still am) licensed in OH in 2003 and moved to CA in 2008. If Illinois uses the NCEES 8hr exam and you passed it, then you will get reciprocity for the 8hr exam only.The 8hr is only part of getting licensed in Cali. California has two state specific exams for SURVEYING and SEISMIC PRINCIPLES. They are 2.5hrs each and you can sit for them the same day. The tests are administered the same saturday as the EIT/FE. You also have to take an at home test on PE rules that you submit with your application. It's open book no time limit and makes sure you have read all the PE regulations (not difficult).
California application is very specific on your experience although they require only two years of it to sit for the exam. The experience must be certain tasks under a PE and the PE you say that the experience is under has to sign off on it. I'm not sure how IL is but OH is basically you wait for 4 years after you graduate and as long as your working and you get some PE references your good to go.
My two pieces of advice.
1.) Start really early on your application and reference letters.
2.) Get Hiner's review book for seismic (It's up to you if you take his class, but it's likely the best)