Each MLS has its own rules (48 hours is common), and those rules apply to only the MLS database. The MLS database is shared via IDX (internet data exchange) with various brokers and third party vendors. Those updates are not real time. Therefore, once an agent updates the MLS system, the same property could appear elsewhere, Realtor.com, just as one example for 1-2 days after the update is made. Additionally, other real estate lisitng sites are not updated by MLS data updates, so those sites could be updated sooner or later. MLS organizations do not patrol any database except their own.
Trulia, for example depends upon agents to update the data or the third party vendor that supplied the property information. This would also be true of numerous other data sources for property information.
If you have a buyers agent, he/she can check the MLS and tell you the status as it appears there.
There might be different rules for each Multiple Listing Services. For Marin County, CA, we use Bay Area MLS. We are required to update the listing status within two business days of status changes. Our MLS personnel actually patrol MLS and the members can flag the listing if they know of any violation. The agents can be fined if they are in violation.
However, some commercial sites will not show status changes until it's either pending or sold. So, many times a status on MLS has changed to contingent or pending but they are not reflected on the commercial sites. So, check with your Realtor on the most current status of a listing.