There is no real answer because it depends on the internal circuit of the op-amp but wdmsystem's formula is correct.
On most op-amps, the signal input pins are connected directly to the base or gate of a transistor so there is likely to be a non-linear realtionship between input voltage and current, this means the 'resistance' could be different depending on how you measure it. On FET input op-amps, the resistance will typically be very high, in the tens or hundreds of million Ohms region but on some bipolar input amplifiers it can be quite low, only a few K Ohms.
Brian.