For starters, please use more descriptive thread titles. "EWR ?" fails to articulate the question you're trying to ask. Something along the lines of "Connection Times at EWR between Term A & C - How Long?" would better capture the question you're trying to ask.

And on that note, this topic has cropped up ever now and then and there's a handy-dandy tool called "search". You'd be surprised how much quicker you can fleece out answers from the collective postings of all the folks here rather than waiting for others to respond.

Moving on, as I understand it, you're connecting in EWR from CHS on your way to LAS and want to know how long the intra-terminal transit will take. It looks like flights from CHS (on ExpressJet) arrive at Terminal C, not terminal A (if memory serves, flights from ATL, DFW, ORD, BOS, DCA are the only ones to fly into Terminal A). The good news is that you'll already be in EWR's Terminal C and that you won't have to make use of any trams/buses or reclear security. That said, your estimate of 15mins sounds about right as all you'll have to do is walk from the ExpressJet "pylon" (that's the one in the middle on the afore linked EWR terminal map) to one of the other ones.

Should you want to stop to grab a bite or a book you should be good too as there are abundant eateries and shops in EWR.

Good luck in Vegas. ^