If the ND value is stated on the filter as ND2, for example, you can simply calculate that the exposure time doubles. So with an ND1000, the exposure time is 1000 times as long as without a grey filter.
However, if the value on the filter is given as an NDx value, such as NDx 3.0, this is a logarithmic value. Here the extension factor is calculated from 10 to the power of 3.0. The result is 10 x 10 x 10 = 1000, which means that the exposure time is increased by a factor of 1000, i.e. you can expose for 1000 times as long as without the ND filter.