Texas is requesting a response from SCOTUS by Dec 10? Or from the 4 named states?
TX, FL and other red states. Are making a clear proclamation that they consider Biden to be illigetimate.
The timeline for response to lawsuits is generally by standing rule - get sued, have 21 days to answer sort of thing.
This is an interesting case because SCOTUS is acting as a trial court, not an appeals court. I don;t believe there is a rulebook for this, unless they use the Federal Rules of Civil Procedure.
https://www.law.cornell.edu/rules/frcp - https://www.law.cornell.edu/rules/frcp/rule_12
The 10 am Thursday deadline is set by order here, an order from SCOTUS to the defendant states.
Without having read any of the filings (been busy with paying work and family issues), plaintiff states will have to prove (preponderance, 51% probability) the election contests in those states are certified on fraudulent returns. The remedy part has one possibility that I think works. Enjoin defendant states from certification either way. Flawed election process. The states can then choose to have their legislatures choose, or send no electors to US Congress.