I have been following this for a few days. While preamps using small tubes will run for years, depending on how they are implemented, they do wear out. Power tubes, not the real issue here, often have short life spans.
The thing is with preamp tubes, you may not notice the change, until you put in fresh tubes. And there are no general rules, some preamps run their tubes hard. There was one preamp that was notorious for tube issues, it went through 6922s very quickly. And when you get into NOS tubes, different years of production has produced different quality in tubes. Buying NOS from Upscale may be the safest place to buy true NOS. eBay may claim a tube to be NOS, but the tube may have been used. There are already reports of early failure, and additional hiss.
I have run 12AX7, 12AU7, 6CG7/6FQ7, and now 6H30. I am also much like the OP, my stereo is on 16-18 hours a day. I can put my current preamp in standby, the preamp that ran 6CG7 and my other that runs 12AU7, were/are left on 24 hours a day. No failures, but fresh tubes always sounded a little better after burnin.
I just replaced the 6H30s after 7500 hours, and will in the future aim for 5000 hours. 6H30s are considered super tubes with a history of use by the Russian military.
The only definitive answer, will be the one from BHK, as he knows how hard he is driving those tubes. Until you get that answer, be conservative, you can always keep the tubes you pulled as backups, available for troubleshooting, or to be used again, if finances are tight.
You spent $6000 on a preamp, even more for a power amp/s I would not follow the 10,000 hr with those components. Although that is just my opinion, I don’t claim to be an expert. I can tell you this, I found a little tube hiss acceptable when running a $2000 preamp, I would not in one costing $6000. My Conrad Johnson CT5, bought used, is dead silent, Audio Research preamps I have heard are dead silent. The new Reference from AR does cost twice what the BHK does, but my CT5 listed new for $6500, and I would have to hear it against the BHK, before I would be able to say which sounds better, but that silent CT5 was a huge step forward in my system.
I think you need to figure out how many hours you are putting on your tubes, and decide if replacing tubes every couple years at around 3000-5000 hours wouldn’t be prudent. And if you find a NOS tube that is superior in sound quality, buy three or four extra sets, as they may not be available in a few years, or the price will have skyrocketed. Want an example, you used to be able to buy NOS 6H30dr for around, I was told a $100 a pair, now if you can find a matched pair it’s a minimum of $250, and climbing. Fortunately they are making new ones. I just bought a cyroed matched pair from Cyroset for $94 with shipping. For currently made tubes check out Cyroset, I got great service and tubes that sound excellent.
I think it time for BHK to give his expert opinion on how many hours he recommends. That will be the only answer that is definitive.