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The modulation type is a first place to start. An FSK system can have a sloppy settling time maybe measured to where the final frequency is within 10 KHz or so. A 128 QAM system would need a very quick settling time, measured to where the PLL is locked and the phase has settled to perhaps 0.5 degrees.
Generally, parameters in the time domain help determine the lock time. For example, in GSM/EDGE, the system is TDMA, so the handset switched back and forth between transmit and receive. The timing on that determines the PLL lock time. In a CDMA system, the handset is full duplex, so the lock time is determined by base station to base station handoff requirements.
For each protocol, there is something specific that ends up determining the allowable lock time, and it's probably different for each. You need to get more specific.