You can set the valid timeframe for the transactions, by defining the timebound in which the transaction remains valid. To enable the policy: 
const session: Session = {
  owners: {
    type: "ecdsa",
    accounts: [accountA],
  },
  policies: [
    {
      type: "time-frame",
      validAfter: Date.now(),
      validUntil: Date.now() + 1000 * 60 * 60 * 24,
    },
  ],
};
This will create a session that will be valid for 24 hours.