Are instructions saying “25 seconds” and “30 seconds” only five seconds apart? Set out the tea and water quantities too, and other differences appear. Waiting time is just the final item in a recipe.
Let us reread the official brewing instructions from Maruhachi Seichajo and Ippodo Tea in the order of preparing a cup.
Before the 25 seconds come 6 g and 260 mL
For the loose-leaf version of Kenjō Kaga Bōcha, Maruhachi Seichajo advises adding 260 mL of boiling water to 6 g of tea and brewing for 25 seconds. Tea bags have a separate method, so match the format as well as the product name.
Remembering only the 25 seconds, then substantially changing the tea quantity or using a different water volume, creates different conditions from the original method. What you are trying to follow is the whole sequence from preparing the tea and water, not just the wait.
Before the 30 seconds come 10 g and 240 mL
Ippodo Tea’s Gokujo Hojicha uses 10 g of tea with 240 mL of water at 100°C, brewed for 30 seconds. Timing starts when you begin pouring the water.
Compared with the Kenjō Kaga Bōcha example, this uses more tea and less water. Looking only at the time difference hides both of those changes. The teas themselves are also different, so a difference in the finished taste cannot be explained solely as the effect of five seconds.
Expressed as tea per 100 mL of water, the first method uses about 2.3 g and the second about 4.2 g. These are values calculated from the original quantities for comparison, not new recipes for brewing these smaller amounts. They show that, alongside a difference of just five seconds, the tea-to-water proportions differ substantially.
General guidance and product instructions serve different purposes
The Ministry of Agriculture, Forestry and Fisheries gives a general guide of 90–100°C and 30 seconds to 1 minute for teas including hojicha. Guidance with a range like this serves a different purpose from instructions for a particular product.
A general guide is an introduction to brewing. When you are about to brew a product you already have, instructions matching its format and quantities provide a concrete starting point. Slightly different figures do not necessarily mean that one set is wrong.
Research also treats tea quantity and time as separate factors
A study of deep-steamed sencha found that changing the tea quantity, infusion time or number of infusions changed component quantities and taste-sensor readings. This was a study of a particular sencha, not a finding of the best brewing time for hojicha.
The approach of treating quantity and time as separate factors is nevertheless useful when comparing recipes. Measuring the intensity of a taste and deciding whether you like that taste can also be considered separately.