My safest answer is: it depends.
It depends upon how seasoned the musician is. It depends upon the setting he's playing in (Is it a few songs during praise and worship service? Or, is he playing a concert?) It depends upon whether you're speaking in terms of knowledge of theory or knowledge of every single note, chord, progression, riff and run in his repetoire. If it's the former, then I suspect that the ratio of what he plays to what he knows will be higher, but if it's the latter, the ratio may even be lower.