Mean holding time and congestion are two different things.
What is the SDCCH congestion at busy hour, for this cell ?
what is the SDCCH Erlang at busy hour ? (8 erlangs?)
If there is no congestion, then don’t worry about the mean holding time : it just means the MS’s are taking a long time to complete the location update procedure. It’s probably because of a slow response from core network (MSC, HLR, AuC, EIR ?)
If there is congestion, then it’s not normal : 8 erl. of traffic / 14.2 of capacity. Check that all your SDCCH timeslots are working and available, check SDCCH erlang and mean holding time per TRX (assuming each SDCCH is located on a different TRX)