From Wikipedia:
"According to Umdat al-Salik wa Uddat al-Nasik, a medieval summary of Shafi'i jurisprudence, hudnas with a non-Muslim enemy should be limited to 10 years: "if Muslims are weak, a truce may be made for ten years if necessary, for the Prophet made a truce with the Quraysh for that long, as is related by Abu Dawud" ('Umdat as-Salik, o9.16)"
As a practical matter, the period would be much shorter - as soon as our troops are out of Afghanistan, the Taliban will be up to their old tricks. Look how quickly things are going to Hell in Iraq since we left!
Etiam non princeps sed usque ad genua, Principis Pacis!