I just found this website today and I may be able to help a bit. I have been investating GSM performance for the sake of writing a class (I work for a training company.) One of the books I have been reading is GSM, GPRS and EDGE Performance by Timo Halonen, Javier Romero and Juan Melero (published by Wiley.) They assume a 2.8 second SDCCH hold time for a call establishment and a 3.5 second hold time for location updates and SMS transfers. I am in the process of seeing if I can verify these because I don’t like these numbers.