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As long as possible. Being able to solve tasks says little about playing skills. When pupils regularly apply what they have learned in their games, they can think about taking the next step. Children who do not capture their opponents' unprotected pieces in their games are far from ready for Step 2. The Manual for Step 1 states the following: "The elementary learning seems simple, and some teachers manage to work through the first step in three months. There are better approaches than this. Essential chess techniques like mating require a longer learning time. It is better to set aside at least a year for the first step to master the basic skills properly (there will always be exceptions). The 'lost' time will be made up later with ease."
The Step 1 plus workbook is an excellent addition to increasing pupils' skills. Pupils get better at chess, which increases the chances of more extended membership.
More and more chess players are switching to solving tasks on a screen. Not only is that a far too limited way of training chess, but that method has disadvantages besides advantages (see the manuals). I will highlight two objections.
On a computer, the student sees the position on the screen after each move. So there is less need to hold the positions in working memory after one or more moves. That skill, indispensable for thinking ahead (important for getting better at chess), must be practised more. This process starts in step 1, and the consequence, especially if the variants are more moves, is highly questionable.
Step 2 adds a skill that is 'automatically' practised when solving on paper, but is less necessary with the computer. That is finding and looking at the answer to the planned move. That's more moves. The computer's countermove is just one and it comes automatically. There is no need to look at alternatives. Superficiality trumps, because real thinking is less necessary.
All 32 books are in English. The higher-step workbooks are all quadrilingual. The reminders can be found at Downloads.