An allotment period shall be one-half (1/2) of twelve (12) months, and expenditure one-half (1/2) of the appropriated amount, unless otherwise specified in the appropriation bill or justified by the agency to the Department of Finance and Administration, and the first allotment period shall commence on July 1. Estimates shall be filed with the Department of Finance and Administration not later than the first day of the month preceding the beginning period. The Department of Finance and Administration may, in its discretion, restrict an agency to a monthly allotment period when it becomes evident that an agency's rate of expenditure to date indicates this restriction will be necessary to prevent depletion of its appropriation prior to the close of the fiscal year or when the condition of the State General Fund requires monthly monitoring and control of the rate of General Fund expenditures.