Before death. See @mattclarke's post for details.
I guess the idea would be the same as with some tribal societies whose elderly sailed off never to be seen again. Matt said it would be voluntary. People who opt to die this way rather than in a hospital bed would wear space suits and be able to observe the magnificient view of the Earth beneath them until their orbits degrade and they begin their re-entry into the atmosphere. Before that, the gas mixture given to them would be lowered in oxygen to make them pass out and die before the going gets too hot.