As far as I'm aware, the certificate for the Economically Weaker Section is valid for the duration specified in the form (it has to be renewed every year). Additionally, if you're thinking about applying to colleges this year, most universities, like DU, allow its students to submit previous certificates. And in some places there is a bar which says that the EBC certificate is valid for a period of two years, so it would be best to confirm from the place where you plan to use it.

Hope I have answered your question. All the best!