Optimal intervention time

[Atlas]

When my DD (ADHD inattentive) works on a problem or problem set, she can spend an infinite amount of time. This can be due to a variety of reasons: spacing out, unable to hold on to parts of the problem at one time, not clear on the concept, or clear on the concept but just takes a while, or just really trying to figure it out. This can happen whether she knows how to do the problem or not. If she doesn't know how to do a problem, she never quits. When we try to set timers, she can end up staring at the timer, and only start working on the problem when the timer rings. Sometimes I would gently remind her of the task at hand and she would get back to work. If she is spacing out, I want to move her along. If she is just really trying to put things together in her head, I don't want to cut that off because that's how she builds up her thinking muscles. On the other hand she doesn't have an infinite amount of time. So what is the optimal intervention time?  

[cintinative]

I would love to hear people's perspectives on this as well.

My youngest is gifted with anxiety and ADD-inattentive.  Sometimes, the inability to focus causes the anxiety to peak. Sometimes the anxiety over not knowing how to do something causes an aggravation of the ADD-inattentive.  The analogy I use is that sometimes the anxiety drives the bus, and sometimes the ADD drives the bus, and sometimes my son drives the bus.  I hope we are moving more toward the latter scenario.