In the key of A there is a natural sign at a G# note in the fifth bar. Is it only for this G# or are the following G's natural as well?
Any accidental — in this case, a natural sign — is only in force until the end of the bar.
Given a key signature that includes G♯, all Gs are sharp unless a natural sign appears. All subsequent Gs on the same line or space and within the same measure are G naturals. After the barline, the key signature takes over again.