What is maximum length of float in c?

Floating point numbers are always stored according to the underlying architecture. The programming language is immaterial, it must use the same representations as the hardware itself, or at least provide an abstraction of it. C does not provide any abstractions for built-in data types.

Most modern computers use the standard IEEE 754 representation, which caters for single-precision (equivalent to float in C), double-precision (double) and extended-precision (long double).