How far over the limit can I go

Without knowing how they measure, you have to (literally) play it by ear.  If they sound good, then enjoy them.  Obviously, you will need to re-tube at some point, so you might consider purchasing a new set of tubes now if it's in your budget.  If you're really curious, put the new ones in for a few days/weeks to see if you notice an increase in performance.  If you do, keep 'em in.  If you don't, put the old ones back in and get more mileage out of them.  

However, you say your amp sounds "glorious."  In that case - I wouldn't change a thing!!

Probably good for another 1000 hours before noise sets in! You can go by the getter flash - when it starts to turn brown that is a sign of old age! 

Horseshoes and  hand grenades. Kinda sorta maybe.

2000 hrs use recommendation is conservative by audio research

you can extend a good 20-25% past that if you are careful don’t listen loud or have had to drive speakers

the problem with arc amps of your ref110’s lineage is that there is no tube failure protection, so a failing power tube can (and often does) take out a bias resistor and maybe scorch a little section of the pcb, needing a minor repair

given the size and heft of the amp, even a minor repair becomes a pain for handle and ship

so most play it safe, replace tubes sooner than later, follow their recommendation

you can always monitor how close to being used up your tubes are by how much the bias current needs to be turned up from when they were new, i track that...