Much of the time taken to acquire a lock is waiting for the ephemeris to be transmitted from the satellite, which contains the exact details of the satelites' orbits. As the satellites transmit at 50 bits per second, it takes between 18 and 30 seconds to receive the entire ephemeris. This represents the minimum time for a GPS lock with no network connection. However, if you do have a network connection, the ephemeris can be downloaded from the internet in a much shorter amount of time; this is how network-assisted positioning works. Aditionally, the ephemeris is usually valid for 4 hours, so if you've had a GPS lock within the past 4 hours the new lock could theoretically be instantaneous.