I thought it was only good for 24 Calendar months (over age 40), but I guess you can squeak nearly 25 months out of it, from the FAA AME Guide: Third-Class Medical Certificate: A third-class medical [FONT=Arial,Arial][FONT=Arial,Arial]c[/FONT][/FONT]ertificate is valid for the remainder of the month of issue; plus
24-calendar months for operations requiring a third-class medical certificate,
24-calendar months for operations requiring a third-class medical certificate,