If a uniform amount of capital is systematically withdrawn by a partner at the end of each quarter, interest on the total aggregate yearly drawings is computed across an average timeline of:

Using the average period technique formula for identical regular drawing patterns:
$$\text{Average Period}=\frac{\text{Months left after 1st drawing}+\text{Months left after last drawing}}{2}$$
Since the first quarter ends 3 months into the year, there are $12-3=9\text{ months}$ remaining.The last quarter ends on the final day of the financial year, leaving $0\text{ months}$ remaining.
$$\text{Average Period}=\frac{9+0}{2}=4.5\text{ months}$$