The definition of cryptographic pseudorandom generators are deterministic functions that turn a true random number (seed) of bit length k into a stream of seemi

The definition of cryptographic pseudorandom generators are deterministic functions that turn a true random number (seed) of bit length k into a stream of seemingly random numbers of bit length n, where n is much larger than k. A computationally bounded attacker cannot distinguish this stream of pseudo random numbers from true random numbers, unless he/she knows the seed. So if the CSPRNG are truly cryptographic secure, you don't need constant stream of high entropy input. You only need enough starting entropy, say 256 bits, and you will be fine for a long time.