Limit the time on an inbound conference call

Hello All,
I have just got running my first attempt at FreePBX and now have a working system with sip trunks using FreePBX 2.11.
I am using it as an office conference bridge and have been able to get that going fairly quickly.
We would however like to be able to limit the conference time, so that if people listening in leave their phone off the hook, at a preset time limit all calls will be cutoff. We also want to be able to limit the amount off minutes used on an inbound ddi (The provider gives very cheap ddi’s, but the trade off is a cap on inbound minutes).

I have seen suggestions that the Dial options available in the advanced settings could be used, but where there are two entries, the second is only for outbound calls and adding L(3600000:3000000) to the first option seems to have no effect on conference calls.

Has anyone got an idea how this can be achieved either through FreePBX or by editing the asterisk files?