Exploring Client Advocacy Strategies: Opt-In Period Ideas

Question for the group: We are currently building out a new client advocacy strategy and one of the ideas that came up was an opt-in period for advocates – where clients could opt in to be advocates for X numbers of years, with the expecation that they complete certain actions during that time (webinars, client stories, etc.). Has anyone tried something similar before? If yes, what was the period of time you landed on – 1 year seems too short, so we were thinking two? TIA! 🙂