Hello Jane,
In real environnment SDCCH congestion will depend on load of the cell ( cell high or low loaded ) and the type of the cell : LAC border, internal etc..
Take a look at this example:
Imagine you define 1 TS SDCCH on cell that got 1 TRX:
Theoriticaly this cell can handle 2.74 Erl of traffic SDCCH at GOS 0.5 but this load is for One Hour period. Imagine at one time, a bus containing 40 people cross this area (Supposition is done that this cell is a lac border) , that is only 8 MS will do LA updating , the others 32 MS will wait and send again request one time again; this could lead to congestion if this cell is not loaded enough on busy hour to hidden the local time congestion.
Regards.