Speidel tanks are often touted as having the micro-oxygenation benefits of a flex tank or even a barrel, but so far I've only done 8 weeks at most of primary + extended maceration in mine.
Someone on this site warned me against using them as secondaries, based on knowing a guy who aged all his wines in them and not liking his wines.
Also, my 30 liter Speidel holds about 9 gallons (34.07 liters), and filling that to the top would be a real challenge, plus the top doesn't narrow as much as the neck of a glass carboy does. For both of those reasons I'd worry about oxidation at the top of the wine.
But here's what Morewine says about them: Can I Use The Plastic Speidel Tanks for Storing Wine? | MoreWine
Can I Use The Plastic Speidel Tanks for Storing Wine?
We would not hesitate to use these for wine storage for 6 months, give or take depending on the kind of wine and the size of the tank (more on that later). Speidels are made from thick HDPE, the same material as Flextanks are made from. For those not familiar with Flextanks these are similar HDPE tanks made for wine storage that have similar oxygen transfer rates to oak barrels.
So the idea is that you can store wine with in these types of tanks with oak and get similar results to the slow micro-oxygenation you get with a barrel. We would suggest for longer term storage you remove the airlock and attach the threaded cap. Depending on tank size, more oxygen transfer will occur through the airlock than will through the sidewalls of the tank.
When we were considering these tanks for sale we were hesitant about long term wine storage as well even though we knew about the material and the science. One real life example was that in Germany these Speidel HDPE tanks can be found in nearly every winery for left over small lots. In a winery we visited in the Mosel Valley they had high-quality Riesling stored in the larger size speidel tanks for a year.
If you are making a wine kit the micro-oxygenation is going to play less of a factor since kit wines are designed not to require long term aging. If you are making a high tannin red directly from grapes you will want some micro-ox in your vessel. If you store in a carboy or stainless tank your wine will go reductive and is more likely to develop sulphur based compounds that reduce the perception of ripe fruit. For big reds where barrels are not an option, the Speidel HDPE tanks are ideal.
With that said the larger the HDPE vessel the less overall surface area. So if you have a larger size tank and a big red wine you can leave it in there longer. If you have a more delicate wine (white, pinot, etc) and a small tank you should leave it in for less time. It would be great if we could say for this type of wine and that size tank here is a rough estimate of months you could store. While we want to work on that more it will always come down to taste as every wine is going to react differently.