Tubes go dim and reach end of life for emissions, and how fast can depend on the heater voltage, runtime of the tube, and manufacture aspects/quality of the tube itself. 2930, 2931, and friends all seem to reach low emissions faster than many other tubes of similar vintage, but it happens to any tube with a lot of use.
If the tube isn't too far gone, then a light hit with a Rejuvinator's Restore function may bring a lot of life out of a tube. If a tube is really far gone, though, then a restoration or even full rejuvination likely won't get more than a couple hundred hours of the tube before it goes back. Your mileage may vary. I've hit some medium-dark tubes with a very light restore and had them run bright and sharp for even years after, while another lasted only a week of daily 9-5 runtime before going dim again. It is worth a try as a relatively cheap option.