Accotrding to NIST 800-90A Table 3, the reseed interval for ctr_drbg should be 2^48.
this means that every 2^48 calls to mbedtls_ctr_drbg_random() with or without additional data, mbedtls_ctr_drbg_reseed()must be called, for security reasons. However, it may be called more frequently.
This is dependent on your product, and what is the expected life-cycle, and your architecture.
You can call the reseed function at startup, or you can call it at other initialization steps.
Other use cases are dependent on whether your application is multiprocess or multithreaded, and there are different entropy contexts for every process \ thread, for example.
I hope this answers your question
Regards,
Mbed TLS Team member
Ron