All experiences are different and your best bet is to check this thread and others like it to get a best guess on dates: http://www.visajourney.com/forums/topic/529349-n-400-january-2015-filers/page-32
You're allowed to leave during N-400 application:
http://kb.immigrationdirect.com/Knowledgebase/Can-I-travel-after-submitting-N400
My suspicion is that you will get your actual letter quite soon. I was "in line" for only a day or so, but my wife's "in line" from the online uscis status website took about 4-5 days. We both received letters on the same day, within a week or so of hers being scheduled (of course online you cannot tell the date, only that they have scheduled it).
If at all possible, you will want to wait until you have the letter in hand and plan around it. You can reschedule the interview, but it will add delay and there is always a risk that they screw something up with the rescheduling.
