To be honest you have no such thing as a right answer to your question, it is always a matter of playing off shutter speed against aperture, or to put it another way speed against depth of field. Even then, if the light is bad you may well have to open the aperture more than you would like!

The problem is you have so many variables, the subject, the background, the aperture, the speed; all will make a difference to the result!

If the day is dull with a lot of cloud, try for pictures with some ground based background, unless that is you have the change of a good close-up, in this case try to get as fast a shutter speed as is possible!

Regards

Eric