How far over the limit can I go

Also usually you can go ten over. But not in Virginia.

I can tell you that catastrophic tube failure can be quite expensive. It cost me $650 and the builder was generous with me because it was not that old. But I bought it used and thus no warranty. I also learned the pitfalls of buying an amp that  has not been broken in yet. Break in can happen either for the good or it  really BREAKS in,  IE failure. BTW It had protection too. But, it still took out a power supply. 

FWIW if you have a catastrophic failure, you will need tubes AND repair. 8 tubes should not cost $1k. Driver tubes  may be alright  since they have about double the life of power tubes. Just a thought.

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.