RETS is not a standard. It's a wannabe standard. It "loosely" defines a schema that describes a real estate listing.
In version 1.x, the "standard" was composed of DTD documents. In 2.x, the "standard" uses XSD documents to describe the list.
http://www.rets.org/documentation
However, in practice, there is almost no consistency amongst implementers. Having connected to hundreds of "RETS Compliant" service providers, I'm convinced that not one of them is like any other one.
Furthermore, the 2.x "standard" has not changed in 3 years. It's an unmaintained, sloppy attempt at a standard. It (RETS) is used as a buzz word by people that don't understand underlying technologies to try and sound smart.
Try asking the specific implementer for their documentation. Usually, they don't have any. So emailing the lead developer has been helpful. Sometimes they'll provide a WSDL which will outline the supported calls. Often, the WSDL doesn't coincide with the actual service, so beware.
As for your specific question, try caching the results. Usually, the use of a limit on a RETS call is a sign of a direct dependency. As requests for your service increase, the load that your service puts on theirs will break (and not be appreciated). Also, if their service goes down (even temporarily), yours will be interrupted as well. Most importantly, it will make the live requests to your pages really, really slow (especially if their system is slow at the time). The listings usually don't change frequently enough for worries about stale data, so caching up to and hour is pretty acceptable.
Best of luck!