I have a mission sub-one lite. I prefer quiet over speed to reduce string jumping. I've had a number of cross-bows over the years and have done the maintenance myself. I was a compound shooter, so for me, it is just an extension of what I had been doing. Some folks recommend changing the string and cables based on time, but I don't. I prefer to change them based on wear. I've found each crossbow seems to wear them at different rates. There is lot less tuning when it comes to a crossbow over a compound. You still have to do cam timing for most. If you are changing the string, that is the time to do other maintenance as you will need to sight it back in anyway. I like to remove cams, clean and lube them.
I think the 2 year thing comes more from the compound world. With a compound, folks need to do a lot more practice shooting as shooting form has a much bigger impact on accuracy. The crossbow physically enforces most of the form with the rail. This means most folks shoot a crossbow much less. The other issue is safety. With a compound bow you are holding the release back at your face. Failures typically occur during the draw or release. I think there is more inherent danger during the draw of a compound bow. Most crossbows are best drawn with a cocking device. There is still a level of danger if the string fails, but it is further from your face.
Look for any fuzzy areas on the string or separation with the serving. These are signs you need to change the string. Keep i mind that if you are doing DIY maintenance, not all bow presses will handle a crossbow. When I bought my press, I new it wouldn't be long before I had to give up the compound, so I made sure I got a press that could handle crossbows as well.
Triflow is a pretty good lubricant for the cam pins. You can also soak them in synthetic motor oil.
As for velocity, you don't really need the chrono. The question is whether the target impact point has dropped. If you have swapped out sights or something, you may not know. Otherwise, if you don't have to adjust the sight from year to year shooting the same arrows, you are not losing enough velocity to matter.
