The thinking behind letting meat rest after you cook it, but before you cut into and eat it, is that a little time will allow some of the moisture that has worked its way out of the meat to begin to seep its way back in as the meat cools.
Letting meat rest is generally something that is done with those that are roasted, rather than those that are grilled or char-broiled, though in some instances, letting your meat rest a moment before you pick up your knife and slice into it, is appropriate.
For the most part, the absolute longest you would ever want your steak to rest is the time it takes the temperature inside the meat to drop low enough so as to avoid burning your mouth and tongue. In general, this time would be anywhere from one or two minutes to up to perhaps five minutes, from the time it has been removed from the heat, depending on the thickness and cut of the meat. For more tender cuts, such as a filet, the time to cool is much quicker and thus lag times are ill advised. For heftier steaks such as T-bones and Porterhouse, wait times would depend strictly on size and weight. A five pound steaks would take at least a couple of minutes longer to cool to a comfortable temperature than would something on the order of just two or three pounds.
Another consideration in letting a steak rest before cutting into it is following simple manners. If you are served first, it is generally considered impolite to cut into your meat and start eating before others in your party have been served. Thus, in this case, the wait time would be nil since what you are actually waiting on is serve time.
Another final thing that might affect rest time for a steak would be your personal state when being served, or even when serving yourself. Factors such as alcohol consumption, degree of hunger and how tired you are may all affect your ability to taste your steak, and thus you may want to pause a moment or two before digging in to allow the juices to soak into the meat, thus hopefully adding a tad more flavor to the meat and making it more palatable for the under eager eater.
Overall though, for most people, letting your steak rest is neither necessary nor required for a optimal steak eating experience as most people have it served to them, and the time it takes the wait staff to answer the call of the cook and then deliver it, is more than enough time to allow the steak to simmer in its own juices.