I didn’t say that we have different Implicit Detach Timer values 🙂 I said that the range from 6 min to 30 hours is allowed in our equipment.
About VLR purge – see my post from 9 Jun 2009. It is fixed value in our equipment.
When MS becames available it performs ordinary idle mode procedures (celection/reselection, periodic location updating). When the first contact with network takes place then MS is marked in VLR as attach once again wit location registration procedures triggering between VLR and HLR.