As far as I know, the Montgomery GI Bill generally expires at 10yrs after your last day of active service.
You can select to continue to receive benefits under it or select the Post 9/11 GI Bill, but you receive whatever months you have remaining under the new benefit program.
If you exhausted the Montgomery GI Bill before the Post 9/11 GI Bill came into effect (August 1, 2009), you can request benefits under the Post 9/11 GI Bill and you can receive up to 48 months of benefits (statutory limit of benefits vets can receive combining all benefit programs). The VA is no longer advertising this, however I took a snapshot (print screen) of the VA's website before they took it down.
If you did not exhaust the Montgomery GI Bill before the Post 9/11 GI Bill became effective (August 1, 2009), be advised, the VA has warned that you receive one of the two programs (either Montgomery GI Bill or Post 9/11 GI Bill - hence the election between the two programs). Recently the VA's website tells you that you can use up the Montgomery GI Bill and then request the Post 9/11 GI Bill. Be careful when selecting one program over the other.
Post 9/11 GI Bill