According to this model, the returns to education take so long that leaders need at least a 30 year horizon to start investing in schools.
"In the context of developing economies, investing in schools (relative to roads) is characterized by much larger long-run returns, but also by a much more pronounced intertemporal substitution of labor and crowding-out of private investment. Therefore, the public investment composition has profound repercussions on government debt sustainability, and is characterized by a trade-o, with important welfare implications. A myopic government would not invest in social infrastructure at all. The model predicts an horizon of at least thirty years for political leaders to start investing in schools and twice-as-long an horizon for the size of expenditures to be comparable to the socially-optimal level."