Not exactly Radar707. A sectors capacity is measured in movements per hour ( as in Balix's example of 42 movements per hour for the galloway sector). If demand for a sector exceeds its capacity over that hour, then a flow restriction is imposed. Therefore by the very nature of this calculation a flow restriction must apply for a minimum of sixty minutes.
However it can sometimes occur ( such as during the morning rush) that a large number of departures are expected through a sectors airspace within a short period of time.( Say 36 a/c in 30 minutes). Although the number may be less than the sectors capacity, the concentration of flights demands a restriction be applied, hence MDI's are used. MDI's are therefore always applied for less than 60 minutes, typically 30 to 45 minutes. This is a practice that has been going on for many years at Southern airfields. It is just that with the growth of traffic North of the border we are starting to see them more frequently here too.