[ This question, Somnus can certainly answer, because the answer itself is simple: ]

Too long. [ As for the exact amount, it doesn't seem he cares to specify. ]

Enough to assume there are temporal discrepancies ...but we must not act as if time away does not jeopardize our Star.

[ The Ring is a tool, but also a conduit. Every single king's soul is to be sacrificed with it. Noctis is the key, but the others are needed, which includes Somnus. Otherwise, the light will not be strong enough to eradicate the Scourge once and for all. ]