Originally it was supposed to be a year but I believe she came back possibly a few months early.
Hahha if it's a year who the dad
My guess is, Cassius because I can't imagine anyone else willing to sleep with her. My theory, on the night of Seria's wedding, him and Lina did the devil's tango but we don't know that as the scene cut back to the Seria's moment
The reason why I state Cassius is that, I don't believe she went back to her original world, no. She was probably in a void where stopped her clock completely until the time was right to put her back into that world hence why she has a flat stomach and unaware she's pregnant.
Another candidate, which is going to be a very long stretch, is Rouche's cousin who looked like him sense he was the ogml but after getting, his appearance changed.
My money is still on Cassius as it makes the most sense.
*getting caught
How long was she absent before falling from the moon?