RSA-230 has 230 decimal digits, or 762 bits.[0]

RSA-1024 has 309 decimal digits, or 1024 bits.[1]

So the relative increase in size in bits is 1024/762, roughly a 34% increase, a factor of about 1.34.

Currently the complexity of the NFS is thought to be:

exp( c (ln n)^(1/3) (ln ln n)^(2/3) ) [2]

where c is about 2. That means that RSA-1024 (1024 bits) will take about 18% to 19% longer than RSA-230 (762 bits).[3]

[0] https://en.wikipedia.org/wiki/RSA_numbers#RSA-230

[1] https://en.wikipedia.org/wiki/RSA_numbers#RSA-1024

[2] https://en.wikipedia.org/wiki/General_number_field_sieve

[3] Program:

    #!/usr/bin/python

    from math import *

    ln = log

    def complexity( n ):

        return exp( \
            (64./9)**(1./3) * \
            (ln(n)**(1./3)) * \
            (ln(ln(n))**(2./3)) \
            )

    print complexity(762)
    print complexity(1024)
    print complexity(1024)/complexity(762)