Late 90s to early 2000s for sure. Tons and tons of backends of various kinds were written in it. If it was Windows it was typically running on COM/DCOM, and in UNIX it was usually CORBA. Sure, the UI was typically written in something else. Dawn of the Web with CGI, and if we are talking desktop people would use something like Visual Basic, Power Builder, or Delphi. But C++ pretty much dominated that space until Java got decent enough and fast enough.