Depending on where the listing is submitted (Realtor.com, Trulia.com, Zillow.com, postlets.com, Craigslist--which the agent has to do personally--etc.), it can take as long as 72 hrs+ for it to show up online. I submitted a listing that didn't show up for 10 days on one of the sites, so give it a week and try again. The easiest way for the time being is to see if it appears on the agent's website or the company's website since those turnaround times are usually shorter for the upload of data to/from MLS. Our company's site updates every 15 min.
Sometimes agents even purchase a separate website for the property itself. The Realtor/their company may/may not have an enhanced listing on Realtor.com like our company does (where you can add up to 25 pics, virual tours, long text descriptions, banners, headlines, open houses, etc.), but the info usually shows up there on that site after about 24 hours if the company has an IDX agreement w/ Realtor.com.
There's a ton of paperwork and marketing materials to prepare for listings, which often take me a couple of weeks...but I work on the internet presence first and then print materials after that since 84% of people search for homes online first.