Hi,
Regular TestComplete/TestExecute (TestLeft is based on their engine) has aqUtils.Delay(<timeout in ms>) method. You may use Object Spy in TestLeft, search for this method and call it. (Sorry, cannot provide you with the more detailed code at the moment.) Obviously, this is a quick and dirty way.
If your tested application allows it, my preference is like this:
-- get reference to the target grid (wait for it);
-- get its current number of (displayed) records;
-- in a loop, with some small delay (e.g. 500ms), for some timeout, wait until the number of records is greater than zero and does not change (which means that the grid has been populated with data).