You mentioned that Sr. Mgs. can't get a Controller gig due to lack of private experience. If that's the case, when's the best time to get out? Manager? Senior III?
It really depends on what you want to do after you leave. If you want to switch career paths, leave now. If you want a 9-5 lifestyle as a senior accountant, so you can leave at a normal time and enjoy life, with the work not as challenging, leave after 2-3 years. If you want a senior revenue accountant position or something more specific that pays well, is mostly 8:30-6 but during monthly and quarterly closes, wouldn't mind working till 8 pm or so, leave after 4 years. If you want an assistant controller position, a revenue manager position or an SEC reporting manager position, leave after your manager 1 year. If you want a director of technical research position or a controller position (both of which aren't readily available), leave as a manager 3 or as a senior manager.