How far over the limit can I go

You can wait till your amp sounds lifeless, but ARC state 2000hrs, idling, with music, it does not matter. Start thinking of re tubing. Better safe than sorry. 

You will get better dynamics and better detail after you retube.

Usually I wait until a tube gets noisy to worry about it, I've never had a power tube go bad and damage my amp. I have had rectifiers go out twice once taking out a resistor another time blowing a fuse. Probably a wise thing to do is get a retube now and use your current tubes as emergency backups as tubes can and go get noisy and the least convenient times.

As I've posted in other threads, I own a PrimaLuna integrated amp that senses when a power tube is about to fail, puts the amp in safe mode - - and tells you which power tube is the culprit.  In several years of ownership that feature has kicked in twice - potentially saving me a lot of grief and money.

Questions for the group - are there any other tube amp manufacturers that offer a similar feature? If not, why not?  I don't pretend to understand how the "bad tube indicator" circuit in the PrimaLuna works - - or the costs involved.  But it has proven its worth to me, and I rest a bit easier knowing it actually works.

Go ahead and order a set of tubes. You're going to need them soon, so why wait? This way you'll have them on hand when the old ones start to go and you'll experience no down time. You will know when it's time to make the change, your SQ will change for the worse.