Write SQL query to solve given problem: For how long did Elly Koss's cystitis last?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT strftime('%J', T2.STOP) - strftime('%J', T2.START) AS days FROM patients AS T1 INNER JOIN conditions AS T2 ON T1.patient = T2.PATIENT WHERE T1.first = 'Elly' AND T1.last = 'Koss' AND T2.description = 'Cystitis'