The casting time (1 quick action) suggests that it is meant to be launched during encounters.
However, can I throw it out, and use it in the first attack of the first meeting of the day?
Expandable weapon (PHB2, p126):
duration: An attack
In one sentence, the weapon in your hand extends without becoming heavy or heavy.
The affected weapon stretches to its target, although it can be used normally. The spell adds 5 extra feet to a melee weapon for a single attack.