Process control is a procedure I encourage my clients to implement. By regularly printing an evaluation image and comparing them to a "reference print" made immediately after calibration and profiling will allow one to get a feel for how a device changes over time. Despite some evangelical promises, all inkjet printers drift over time and will need either re-calibration or re-profiling (if re-calibration isn't possible). HP and Canon large format inkjet printers have a built-in calibration procedure that, if followed, allows the user to use one profile for the life of the printer. Epson inkjets are very stable for low volume users and often don't require recalibration or re-profiling. However, one thing I've noticed with my high volume clients is that Epson's really do change a lot under high usage and do require re-calibration or re-profiling (the later being the easier option for some users). Under heavy use, you'll notice that evaluation prints off an Epson get darker and the shadow details start to plug up. One can recalibrate in a RIP or Colorbase or simply make a fresh round of profiles. I'm finding it smart for my high volume Epson users to do so once a year (or twice if they are really picky). I'm finding that low and high volume Canon iPF users should recalibrate twice a year or immediately after any parts replacements. It's just so easy to run the procedure on these printers that it's important to take advantage of it.
Silver halide machines are much more volatile and need recalibration anywhere from every 8 hours to once a week, depending on the machine and the users level of pickyness. Again, implementing a process control procedure takes the mystery out of what's happening and allows the user to visually see what's happening and make an educated decision as to how often they should take the time to re-calibrate.
With process control one can profile once, and calibrate often to maintain consistency. For those that don’t like the thought of process control, calibrating silver halide devices every 8 hours, HP and Canon inkjets twice a year, Epson inkjets once a year for high volume and Epson inkjets every 3 years for lower volume usage are decent rules of thumb, IMO.