What is the length of the blues progression in this excerpt?

In blues musicians also often refer to chord progressions using Roman numerals, as this facilitates transposing a song to a new key. For example, rock and blues musicians often think of the 12 bar blues as consisting of I, IV and V chords. Thus a simple version of the 12 bar blues might be expressed as I/I/I/I IV/IV/I/I V/IV/I/I. By thinking of this blues progression in Roman numerals, a backup band or rhythm section could be instructed by a band leader to do the chord progression in any key. For example if the band leader asked the band to play this chord progression in the key of C Major, the chords would be C/C/C/C F/F/C/C G/F/C/C.