Purdah starts today, 30 Mar 15, to coincide with the dissolution of Parliament.
I believe post election the party with the largest majority has 3 weeks to form a Government, which theoretically means no decisions between now and the end of May.
However, given the SDSR, even assuming the CS have already started background work in line with party policies, coupled with the summer recess which last year ran late Jul - early Sep, I would hazard a guess at no major substantive decisions on routine running of Defence, procurement and policy before mid Sep once everything is taken in to account.