Can anyone give me any idea on how many hours generally a ECC85 is good for? I suspect there are a number of variables, but I am interested in finding out what the general average life would be, as I am planning on purchasing some NOS. One last question; any experience with either RFTs Valvo tubes, specifically ECC85s?

Thanks and have a great Holiday.

What does it do? Is it a single triode or like the ECC8x s a twind triode or double 9 pin miniature. I am not familiar with it. What purpose does it serve?
As a very broad generalization and mindless assumption, I will venture that unless it is run real hot beyond it's true limits. Virtually all preamp tubes are low wattage despite being high gain or mu in some instances. An example is a 12AX7/ECC83/E83CC.
It should last the "usual" 10,000 hours.
In the least most preamp tubes typically last 5,000 hours.
The problem is just not one that afflicts the current owner of tube gear. Many of us will have moved on to a new piece of gear or roll the tube before it actually burns out.
The rest of us who have seen this actually happen such as myself are trying out a tube that was bought as testing good. But lord knows how any hours have been put on it, prior to you getting it. The others have tube eating devices or are very, very patient and staying with a piece for twenty years without bothering with retubing it.
Just don't ask a person with a Sonic Frontiers preamp how long a tube will last. If they dare try the less than very durable tube that SF insists you use.
All this stuff is simply, word of mouth, the stuff of legends, barring by the claims made by original tube makers. These claims about how long a tube should go, were made in "wars" over about who made a more durable/reliable tube than the competition. All dates back a number of decades.
My honest advice is to look it up in the RCA (receiving tube) manual.
The web is also replete with reprints assembled by nice people who have the factory insert data or other sources. Generally places like "Franks" data sheets provides a schematic of the tube's circuit, its pin-out, response curves, substitution tube types, and a short page or so describing in engineer speak what the tube is useful for.
In plain speak, look it up, or ask your question on tube asylum of audioasylum.
They love obscure tubes. The more obscure it is, the better it is. In fact they are allways better than the ones those foolish amp designers are still using on a regular basis in commercial amps. In most cases this leads you to the DIY asylum where everyone is an engineer of tube amps.
Good luck and cheers.