In my opinion, the optimum value for SDCCH mean holding time should be around 4s. However, the actual value depends on several factors, including:
1. Radio network features (eg, assignment to other cell)
2. Abis Transmission method (satellite sites will have a far higher MHT than sites on microwave).
3. TCH Congestion
For those cells with the high MHT, check for faulty TRXs and TCH congestion. If the other KPIs are also bad, then it might be an indication of a faulty hardware.
Also check to see the kinds of activities using SDCCHs in the cell. If you have many SMS and/or location updates in the cell, the MHT will be high.