Proposals to reduce the ERASMUS time are stupid.
ERASMUS figures do not decline because of the length of the stays but because home universities too often put too many administrative and educational restrictions on those who want to leave. If leaving 5-6 month means to repeat one whole year of studies, this is a serious negative incentive not to go.
Stays of less then 5 month are too short to connect to the country and culture where the ERASMUS students are going. Building relations to other Europeans - local and international - needs time, especially if these relations should contribute to a European identity, in my eyes one of the main reasons to organise ERASMUS exchanges.
So don't shorten ERASMUS, but ease the exchanges by better inter-university co-ordination and good-will on the side of university administrators!