Depends on what the reason for doing the IR test in the first place ie is it for predictive planned maintenance or fault finding. Also could depend on the motor itself and on the method of supply ie whether there's a VFD or whether DOL. Obviously it would also depend on the motor supply voltage ie is it an LV or MV motor? To complicate matters further many facilities and/or companies also have their own testing requirements that are set in stone regardless.

Most insulation testers use a DC test voltage. If the motor is supplied with 400VAC DOL for example then the peak waveform voltage is around 566V so I'd say the IR test voltage needs to be as high or higher than that value to be meaningful. Depending on the instrument being used the next test voltage available would often be 1kV so therefore that's what I'd use. The regs require a test voltage of double the supply voltage so this would also suggest a test voltage of 1kV.