By John S. McClenahen Senior executives counting on a 10-year run in office would be well advised to count again. Executives and managers have seen job tenure shrink by a gut-rending 36% during the last eight calendar quarters, reveals a survey from Challenger, Gray & Christmas Inc., a Chicago-based outplacement firm. Seven years was the median tenure among executives and managers who lost their jobs between July 1 and Sept. 30 of this year, compared with a median tenure of 11 years in 1998's fourth quarter, an eight-year high. Ten years ago, the median tenure was 13 years. Contributing to dramatically shorter tenure is a sharp rise in the number of executives and managers losing their jobs within two years. Data from 3,000 people show during the last three calendar quarters, 16% were dismissed before their two-year anniversaries. In 1990, less than 8% of executives and managers were let go within two years of being hired.