Hello Aspirant,

As there is no any limited validity time period is not defined by Government of NCT if Delhi so it is hard to tell you the exact validity period of your ST certificate, but you can ask about the validity from the officer issuing you the certificate and also note that caste validity certificate is also mandatory along with ST caste certificate and for the same you can apply by visiting Tehsil or SDM office of your area and ask for the application form for caste validity certificate for ST category and then after filling it submit it to the designated officer and it will take around 1 month for issuing of caste validity certificate.

For more details you can refer to the official site as given below :-

http://revenue.delhi.gov.in/wps/wcm/connect/doit_revenue/Revenue/Home/Services/Certificates/SC-ST+Certificates

I hope this information helps you.

Good Luck!!