User blog comment:Darkcloud1111/''The Mark of Athena'' Plot Holes/@comment-3966122-20121021164750/@comment-3146930-20121021170031

The best way we can say it is 6 months. Percy made a mistake when he said 8 months, as he was only missing from mid-December to about mid/late-June. Rick tries to fix this in The Mark of Athena by Annabeth saying that they had been dating for about 4 months (August to Decemeber). So to answer your question, Percy was missing for 6 months.