I take it your age is between 50 and 60 years old?
My Class 1 medical form states, (regarding expiry dates):
"Class 1 single pilot commercial operations carrying passengers" (date = 6 months hence).
"Class 1 other commercial operations" (date = 12 months hence).
"Class 2" (date = 12 months hence).
So, unless you are single pilot and your students are passengers, I reckon it's valid for 12 months.