Once correlation of the two codes is achieved with a delay lock loop (DLL), it is maintained by a correlation channel within the GPS receiver, and the receiver is sometimes said to have achieved lock or to be locked on to the satellites. If the correlation is somehow interrupted later, the receiver is said to have lost lock. However, as long as the lock is present, the NAV message is available to the receiver. Remember that one of its elements is the broadcast clock correction that relates the satellite's on board clock to GPS time, and a limitation of the pseudorange process comes up.
Once it is achieved, the delay locked loop sustains the correlation to continue. Once the GPS receiver is locked on to the signal and has its code correlated, it can remain locked as long as there isn't a cycle slip, meaning, as long as there is no interruption of the signal from that satellite, it's not obstructed in any way, the receiver can continue to log the signal from the satellite and stay correlated with the delay lock loop.