I guess time can be an issue, but the proof is in a leak down test. Simply put the most can be gained in the 206 with a good sealing valve. In KPX we lap valves every 2-3 races. Many times if the engine is apart already every race. I admit that to be excessive. At club level lapping the valves at half season should keep you very competitive. It may be that your driving skills and Kart setup skills can help much more than the focus on the engine. At higher level where everyone is fine tuning their karts, every three races max. The courts out on the valve springs and different engine builders/ mechanics have there own opinion on that. I think replace at half season.