It depends on how close you are:
Vanilla Ice got busted for plagurism with Ice Ice Baby, Godsmack gets away with "borrowing" Alice in Chains riffs. The legal definition isn't strict as my personal definition, I know that much.

Of course, everybody has influences. Nobody who's listened to me play hasn't thought "that guy listens to Primus." On the other hand, they can't say "Wait a minute... THAT'S 'SEARGENT BAKER!' THAT BASTARD!"

Really though, this isn't an easy question and everybody feels a little differently about it.