I teach theory during the piano lessons, using the ABRSM syllabus. Weekly workbook assignments normally take 5- 15 minutes, separate from practice, and the student can normally progress at their own pace.
I don't enter students for theory exam unless they request it, except for the Grade 5 theory exam, when it is an obligatory prerequisite to take the Grade 6 practical exam.
I find studying theory works well when the student's theory level is always a little behind their practical Grade. This means the student working towards Grade 3 practical exam would ideally be working through the Grade 2 theory syllabus, or perhaps beginning the Grade 3 syllabus.
I can teach theory as a separate subject, but I would discourage anyone from undertaking a formal ABRSM-style theory syllabus at a level much higher than their practical level, and certainly not without taking practical lessons at all. I would however happily teach a more flexible, personalised theory syllabus, without exams, to anyone wanting to enrich their general experience of music.