When the proclamation is drafted to prorogue, a pro forma date must also be set at that time as established by the Prime Minister. Generally, traditionally it has been 40 days or thereabouts, but in the past few decades that has varied. The pro forma date can be changed. Doing so just requires the issuing of a subsequent proclamation.
Over the last three or four decades, it has varied from 14 days in 2002 to 32 days in 2007 to 63 days in 2009. It really depends on the circumstances of the day and the decision. It's the PM's decision.