I'll base the scenario of this question on the PhD thesis defence committee recommendations in place in my university in Ireland. Similar may apply to other countries.
A PhD Defence/Viva committee in made up of a Chairperson, internal examiner and external examiner. The internal comes from within the university and the external from another institution (with some restrictions on affiliated universities).
Assuming for this question cost of travel is not an issue as it would be covered by your university and you/your supervisor agrees with choice of potential external examiner; is there any rule of thumb on how far you could reasonably expect a external examiner to travel to be on the committee?
One downside I can think of asking someone to travel a large distance is the possible time they have to take away from their work. Again taking Ireland as an example, an external examiner from Ireland/UK/EU could probably travel on the day or 2 max. If it was someone travelling from the US they may have to take a few days off. Areas with poor air connections may mean longer trips.
So the question is largely two fold. Should how far away a potential external examiner factor into the decision to ask them to be on the committee and how far you could reasonably expect a external examiner to travel to be on the committee?