Well the convention was to store it as a 32 bit signed integer, so that is any number from -2^31 to (2^31 - 1). Prime numbers are formally defined as a subset of whole numbers, so let's ignore the negative numbers and the number zero.

Fun fact: the largest signed 32-bit integer is itself a prime. And the wikipedia page lists it as the 105,097,565th prime.

By the time we hit the 2038 problem, there will have been about 105 million seconds since 1970 where the Unix time was a prime number. And it's a 10-digit number in base 10, where prime frequency is something about 4% of the numbers.

Does that answer your question about prime frequency today? Eh, I'm sure someone else can figure that out. If not, I'll probably have to wait until I'm in front of a computer.