What are the key indicators that suggest its time to re-evaluate your bike fit, and how frequently should riders reassess their fit to ensure optimal performance and comfort? Is it based on mileage, time spent on the bike, or changes in fitness level or riding style? Are there specific physiological or biomechanical markers that can signal the need for a fit adjustment, such as changes in power output, cadence, or muscle imbalances? Should riders be proactive and schedule regular fit checks, or is it better to wait until they experience discomfort or performance issues? What role do factors like flexibility, strength, and core stability play in determining the frequency of bike fit evaluations, and how can riders incorporate exercises and stretches to maintain optimal bike fit and performance?