The appropriate amount of groundwork for a yearling each week is considerably less than many owners assume, and the temptation to do more — because the yearling is learning quickly, because sessions are going well, because there is always more to teach — should be resisted in favor of what the yearling's developing body and mind can sustain without accumulating stress.
For most yearlings, three to four groundwork sessions per week of fifteen to twenty minutes each is an appropriate workload. This provides enough repetition for learning to consolidate — skills introduced on Monday are remembered and built upon on Wednesday — without overloading the yearling's physical development or its attention and stress management capacity. Yearlings that are worked too much too early develop physical issues from loading growing joints and soft tissue before they are mature enough to handle it, and behavioral issues from chronic stress that shows up as dullness, resistance, or anxiety.
Session quality matters far more than session quantity. A twenty-minute session in which the yearling makes clear progress on two or three specific skills, ends while still engaged and willing, and leaves the yearling in a positive emotional state is worth more than an hour of drilling that pushes the horse past its attention span and ends with a frustrated horse and a frustrated handler.
The off days between sessions are not wasted time — they are when consolidation happens. Research in equine learning consistently shows that young horses show better retention of skills learned in spaced sessions than in massed practice, which means that the two or three days off between sessions are actually contributing to the learning, not interrupting it.