Access-challenge timeout on IOS

Hi, I'm experimenting with a system involving an access-challenge to a NAS. It works fine with FR so far on, say, the cisco ipsec vpn client, which waits a long time until timing out waiting for user input. I'd like to also discover how other NAS's behave using this and have found the timeout on a particular cisco 1131 access point to be quite short. Does anyone know if there's a radius attribute I can send that will extend this timeout, or an internal setting that will change the default on the ap? Session-timeout and Idle-timeout are attributes mentioned by the cisco docs but neither of these seem to be what I'm after. Thanks Andy