Many doctors will tell patients with a positive herpes IgM test that their test result means they were recently infected with herpes (HSV). This is because HSV IgM antibodies are thought to peak shortly after an initial infection and then recede.
In fact, that is the common wisdom for IgM antibodies in general. Compared to IgM, herpes IgG antibodies develop more slowly. However, they are expected to remain high throughout the course of an infection.
Therefore, a positive HSV IgG test is thought to mean that the detected infection is at least several months old.