Difficult to answer because its a VERY individual thing....(and doesn't always equate neatly to health/age/fitness) . Given the shortness of breath you experienced in Mexico City (7200 ft) then Quito at 9000 might be a problem.... the usual advice applies I suppose... stay hydrated, don't exert yourself, some recommend increasing your carbohydrate intake... and take aspirin if that's not a problem for you........shouldn't take more than a day or two to reach a reasonable level of acclimatisation....