I don't believe so, but why would you even need to? Surely an unanswered call will go to voicemail before you reach the end of a 30-second ringtone...
If you need to hear your ringtone for longer than 30 seconds maybe you should just fire up the ipod and play the whole song that way?
People's ringtones are annoying enough. More than 30 seconds and I'd have to start smacking people for waiting that long before they answer the phone.