First, context: the idea that a two-week duration in the low season is a budget constraint is a bit of an overgeneralization.
Here in the UK, for example, the low season can actually offer longer stays for the same budget, thanks to last-minute deals.
It is not the low season that sets the duration, but the local market and its willingness to fill planes or hotels.
For example, a one-week stay in the Canary Islands in February can cost the same as a two-week stay in November, precisely because operators want to maximize occupancy during that period.