Since there are so many variables that can affect yield there's no one answer to what kind of yield should we can expect. What we do is find methods to quickly determine if the material is worthy of being pressed. One of the most basic things that affect yield, is the starting material. Material that is low in quality will reflect that in yield if it yields any at all. So starting with a high-quality material helps yield far better than any other tip we've found. Even great starting material we've found some cultivars just don't work well when pressing rosin.
Pressure seems to be one of the biggest factors that affect yield. The reason we believe pressure plays a huge part is due to the difference in yield between the hair iron and a rosin press. Just by switching to a shop press with our rosin press plates we saw yield increases up to 20%. Using the hair iron to press rosin we'd be lucky to yield 5%.
Heat seems to plays a small part in yield too. We believe the heat reduces the viscosity of the oil allowing it to flow out of the material more freely. Temperatures around 240f seem to yield the best but the quality suffers as the higher temperatures degrade the oil. The opposite seems to hold true when using lower temperatures too. Temperatures around 175f we've saw a higher quality oil but our yield suffered.
Moisture also plays a big role in yield. Herbs too dry will have an unfavorable yield, while herbs with a high moisture content can cause the oil to be contaminated by said moisture. Finding a proper moisture level for our material can really make a huge difference in yield and quality. We found it's best to press our material while it still contains about 65% of its moisture content. You can read more about how moisture affects yield here.
Since every situation is different there's not an estimate to comfortably determined expected yields. What we have is a scale for desirability. We assume 30% is around the max return we'll receive from dried herbs. If we press dried herbs and our returns are under 15% we choose a different cultivar to press unless we believe the quality makes up for the lack of yield. When we're pressing extracts the yield is largely dependent upon the number of contaminants in it. 5-6 star extracts can yield 90%+, while heavily contaminated extracts will be lucky to see 50%. Overall the starting material seems to have a major effect when pressing extracts.