On a regular basis: none. If you have to do it, because of scheduling or timezone conflicts, have the common courtesy to schedule them well in advance. People

On a regular basis: none. If you have to do it, because of scheduling or timezone conflicts, have the common courtesy to schedule them well in advance. People do have lives outside of work, and arbitrarily plunking a meeting a 7 AM or 8 PM with little lead time is disrespectful of their time and commitments.