It depends on the individual. I have taught severe attention deficient students before and you absolutely cannot get them to sit down and focus like normal students do when you want them to. It is a different type of lessons I tend to see their attention comes in waves, almost like waves breaking on a beach, you have to grab them when they have that attention and ride their wave but it is short lived and once it crashes to shore you have to search out for the next wave of concentration. How you wait out and the frequency of these waves depends on the individual. How you transfer knowledge to them also must be different, if something doesn't work abandon it immediately and try another approach, if they fail at trying to correct something too many times it will confuse them and their attention will reduce to zero very fast. With ADD students you must be wary of this much more so than other students. I have found that often how to correct their problem is rather more personalized than a normal solution. What I mean is that normal students often can understand something via the teachers way in solving the problem, but a student with ADD or autism even often rely on understanding solutions to problems via their own perception (this is not to say that "normal" students do not do this as well, but I find that the ADD students tend to learn more from their own solution than being told how to think). Thus you have to ask these type of students a lot more what they are thinking and what they are feeling when faced with problems. The way they describe their problems is key to the teachers method of tacking their problems.
For example. One student of mine was having problems with syncopation and could not understand it, his solution was to see it in a mathematical solution, that is draw out the dots with exact spacings on a graph, then measuring the exact way in which they fall in-between and together with each other. He then went so far to write an algebraic proof to describe x notes against y. We spent an entire lesson doing maths in a music lesson, but it helped this student a lot more than anything else I could have done via my own concept of solving the problem. I applied his solution to the piano and guided his attempt of playing what he read from his graphs. I helped to guide his ear to hear what he was playing via his own mathematical graph of the syncopated notes.
This example was rather complicated often the perception is a lot more simple. If they have a problem with their fingering (technical/memory/musically) you should ask them to describe what they are feeling and where the problem happens. Just making them aware of it and getting them to describe it in their own words will make them start solving the problem on their own. You will find often they cannot describe the problem this is where you have to try to edge them into determining this.
The most severe student I had probably had only 10 minutes out of 30 where he could focus (Often it was only 1 minute focus, then 2 minutes of distraction), the rest we talked and mucked around on the piano just play through pieces they know or are learning with you without stopping to correct them, you may say a few correction but just do not stop them they will take note of the few comments you say when they are mucking about on the piano if you get to know their thought process. You merely make them aware of the mistake but let them carry on, then when you get them to repeat the whole piece again you continue the process but this time maybe ask them to replay when they make the main mistakes.
Attention to getting them to a starting position as quick as possible to repeat the passage to correct their mistake is very important on the teachers behalf, if they take too much time trying to get to the point where they can repeat their passage their interest will wane very fast. Understand which bars the student can start playing their piece from, make inroads into the piece so they can start inside the piece closer to their mistake points so that they don't always have to start from the beginning of the piece all the time. For a student with ADD this is critical as if they have to start from the beginning all the time you may never solve their mistakes since they lose interest when they get to their mistake and just want to keep playing. So focusing on getting them to being able to start at a point close to their mistakes is a good lesson for them and will help you with the limited time you have for their attention in the long run.
We must not forget that our students are actual human beings
We cannot mentally exhaust them, drive them like a bull through a field what have you, for the small time we have them as our students. We need to take notice of their behavior, signals which show when they are losing focus or interest. You notice when they lose their attention, they shift uncomfortably, they face changes expression, they eyes look void or uninterested etc etc, some express their lack of attention by looking around, fiddling or other mannerisms, you have to watch out for it and when they have lost their concentration get them to do something they enjoy to do to recharge them and prepare them to another focus burst. BE wary that not all mannerisms highlight total lost of focus, some just highlight a struggle or strain as they apply themselves, you have to know how far you can push into that, and always push just that little extra but not too much, it will help with their developing focus.
Not everyone needs to learn the piano to become a professional or A grade student, you work with what each student has and do not measure them up against a model you want all your students to abide by. I do have to say that once you get to know your students challenges they will always improve in some form or another, so if you do not see baby steps in their improvement with their piano concentration then you are either setting the bar too high for them or misunderstanding their thought process.
Topic: Adult ADD