Meld into Stone: More than once?

After reading this spell, I couldn't find a clear answer whether one who casts it can meld in and out of stone during the full duration (10 min / level), or if they can only meld once and when that caster steps out of stone the spell is over. Anyone have a ruling on this?

Thanx.

I would rule that because you must meld into stone immediately after casting the spell, once you exit the stone the spells effects end.

Ditto.

Meld into stone enables you to meld your body and possessions into a single block of stone. The stone must be large enough to accommodate your body in all three dimensions. When the casting is complete, you and not more than 100 pounds of nonliving gear merge with the stone.

Casting duration ends, you meld once, into one piece of stone.