The spell's concentration requirement still applies for the duration.
"For every 10 experience points sacrificed by the caster, the creature will remain for an extra day past the spells normal duration."
Normal duration is concentration + 1 round. With the feat, duration is concentration + 1 round + X days?
Edit: If not, a normal summon nature's ally 6 provides a huge elemental. That will also work (though not as awesome).