I think I heard trunk formation on... dicksonia antarctica was about a foot a decade, but it sounds like it can vary a lot depending on how favorable the conditions are. If yours already has a one-foot trunk I imagine it goes faster for cyatheas.

I wish I was in a place that was warm enough to growth these outside. I just have a bunch of ostrich ferns taking over my yard. They kind of form trunks, but they also grow in huge masses so it's not like you'll ever see them.

I do have one in a pot right now. I'm going to experiment on that, try root-pruning or restricting the roots or something and see if I can't get the rhizome growing more vertically.

And if we're talking about ferns surviving adverse conditions, the potted ostrich fern went five months without water, and three months of generally below freezing temperatures, and it's got new growth after getting hit with a few days of rain.

Ferns scare me sometimes.

Under optimum conditions (all-year growth without defoliation) Cyathea cooperi can increase trunk height by up to two feet a year. They are as fast developing as any tree fern you're likely to encounter - in contrast to Dicksonia antarctica - although Cyathea medullaris can also be very quick growing. In the UK, where I grow many tree fern species, that kind of growth rate is only possible under glass. But even outdoors in marginal California you should be able to achieve about half that rate of trunk development - more if you feed very regularly during your summer watering regime. They like LOADS of water in the hot months, and - by tree fern standards - are fairly gross feeders. High nitrogen is what they require - so stay clear of tomato feed, which is rich in precisely the wrong nutrients. Any general purpose fertiliser, well diluted, will be suitable during the peak growing season. But then lay off completely during the winter close-down.

You're fortunate to be able to keep sub-tropical Cyatheas alive through the kind of winter minimums you describe (none of my sub-tropical species would tolerate 'sub-zero' temperatures of that severity). Our wet and mild winters in the UK are statistically warmer than your Californian absolute minimums, yet most gardeners here would still be unable to have Cyathea cooperi outside if there was a risk of frost. But then our two climates (both notionally USDA Zone 9 or 10) have very different implications for what we can, and can't, grow. Cyathea cooperi are still very uncommon indeed on the British fern scene.

If you look close at my photo you can see 2 tree ferns I planted from a 3 gallon pot about 10 years ago.

Here is a link that might be useful: tree fern

I have a Cooperi with a 1ft trunk in a pot in my garden in yorkshire england. As long as its somewhere frost free it seems to be happy. Hasnt lost any fronds this winter, its currently dorment and two fronds are in suspended animation. Give it another couple of weeks once frost is over and I suspect it will start growing. I have an irrigation system set up and it gets watered in the pot and in the top of the trunk once a day and it will get HSK tree fern fertiliser once a week poured into the trunk. They are hardier the bigger they are even though most information you hear about them are that they wont survive been frozen. the bigger the trunk the more insulation the growing point has. As long as its frost free and covered in extreme cold weathers it will be fine