Its nearly impossible to tell.

There are too many variables at play...pollution, soil texture, moisture, pests, disease, climate, the list continues.

300 years is a drop in the bucket for some pines......Great Basin bristlecone pines (Pinus longaeva) are some of the oldest living trees in existence with some more than 4000 years old. And a spruce in Sweden was reported to have been seeded at the end of the last ice age and was determined to be 9500 years old!! Conifers can live a very long time, if in conditions they like and without competition.

Japanese black pines tend to be long lived trees. Not too many venerable specimens left in Japan any longer - all chopped down long ago for timber, etc. - except as bonsai specimens. These are often some of the oldest bonsai plants, many dating several hundred years.

The cultivar 'Thunderhead' hasn't been around all that long so you are not going to locate any of an advanced age to verify. But if given good growing conditions, avoidance of insects and diseases and barring any acts of God, there's no reason to think it will not outlive you and me by any number of decades. Being a dwarf will not necessarily have any bearing on projected lifespan.

I was wondering if dwarfs had shorter lives. Thanks

Not likely shorter lives, no. Now some cultivars can be more tender(or ragged with age), but most just keep on ticking and ticking, well after we're pushing daisies.

One of the oldest conifer 'cultivars', probably the first is a Picea abies specimen, 'Clanbrassiliana' is somewhere between 200-300 years old(or more?). I believe the original plant is both still alive and still growing somewhere in Europe!

-Will

'Thunderhead' has been planted in this area long enough to see that, like so many "dwarf" conifers on the market it picks up steam after being in the ground for awhile, starts to grow out of character. Do not count on it staying under 10'.

Part of the problem is grafting onto non-dwarf seedling rootstocks, which appear to impart vigor not seen in the original seedling or sport. Descriptions given may sometimes be based only on how the original seedling variant grew, on its own roots. With commercial literature in particular there is definitely a tendency for depictions to be made using the growth behavior of comparatively young specimens - same as with other types of trees and shrubs. Older (50 years plus) plantings may contain examples 10 times the height of that given for a cultivar, even in a serious reference manual.

But, that would be a freak occurrence if it was to grow 10times larger, that would be 100 feet, and it would not be a dwarf so the dwarf concept would have failed. It could get taller, but it can't get too wide, because I have limited horizontal space. So, I am more worried about width then height, but maybe I could prune it a bit? It's not drought tolerant, and I give it average water. California summers are very dry. So, it looks fantastic and fresh.

@tropical_thought: the term dwarf does not imply a diminuitive final size... rather, it indicates a "dwarf" growth rate. The dwarf concept has not "failed" simply because the plant gets bigger.

If a specimen lives for a long time, it can still get quite large. Size estimates are generally given at 10 years. As you can imagine, at 30 years it will be 3 times larger.

Here is a link that might be useful: DWARF CONIFERS EXPLAINED