A motion for summary judgment must be served at least 21 days before a hearing on the matter. Whenever that hearing is set (the date), the responding party must file a response at least 7 days before the hearing.
You should contact the court and ask when the hearing (either oral or non-oral) has been scheduled and file your response no later than 7 days before that date.
This is pursuant to Texas Rule of Civil Procedure 166, which you can view here starting on page 61:
http://www.supreme.courts.state.tx.us/rules/trcp/trcp_part_2.pdf
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Attorney and small business owner with 8 years experience in the general practice of law.