Shakuni is the first of the four fixed karanas and it occupies exactly one position in the lunation, position 58, which is the second half of tithi 29, Krishna Chaturdashi. It never occurs anywhere else and it never occurs twice.
The fixed karanas are usually explained by their character. The arithmetic explanation is simpler and is worth having alongside it. A lunation holds sixty karanas. A rotation of seven cannot divide sixty, but it divides fifty six exactly, eight times over. Removing four karanas from the sequence leaves the seven-fold rotation running whole from position 2 to position 57 with nothing broken across the boundary. The four fixed karanas are what makes the seven-fold cycle close. Any other count would leave the rotation cut short at the new moon and starting mid-cycle in the following month.
Where those four sit is then a matter of design rather than necessity, and the scheme puts three of them at the very end of the waning month, at positions 58, 59 and 60, and one at the very start of the waxing month, at position 1. Shakuni, Chatushpada, Naga and Kimstughna therefore form an unbroken run of four half-tithis straddling the conjunction of the Sun and Moon, roughly two days in which no movable karana falls at all.
Like every karana it averages 11 hours and 49 minutes and varies by two or three hours either way.