Health insurance is a big classification, which is subdivided into medical insurance and critical illness insurance. When you say valid time, you should mean after the waiting period. Because strictly speaking, health insurance will take effect only after the insurance company collects the underwriting fee, but it will not claim compensation during the waiting period, only the premium will be refunded.
If you buy medical insurance, the general waiting period is 30 days, and the million medical insurance is 90 days.
If you buy critical illness insurance, the general waiting period is 180 days, at least 90 days.
The function of waiting period is to prevent diseases.
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.