I was able to calculate the package delay values in pico seconds by using the IBIS models from the xilinx site.
For example:
For a Virtex-5 FPGA IBIS model, the package ff323_5vlx20t_ibis.pkg is provided. Let us choose the pin B9.
From Capacitance matrix, C=1.58E-12 F/package length
From Inductance matrix, L=1.99E-09 H/package length
Delay= SQRT(C*L)=56.1 ps
Now I want convert this delay(56.1 ps) to package length(mils).
What method I need to follow for the same?