There is no last date as such for applying GMAT. It is conducted throughout the year. You have to visit the websites or get in touch with the admissions team of the particular schools you are planning to apply to. Normally GMAT scores are valid for 5 years, but many reputed B schools of India have a one year to two year deadline. Like I said above please visit the schools websites.
I hope the above answer would help you.
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