Near the end of the school festival, Shirogane said he's going to leave the school a half year early after the second year to get a head start in Stanford. That's why he confessed to Kaguya, because that would be the last school festival they can enjoy together. Kaguya then agreed that she'll go abroad with him, but as you said, that's still undecided as far as the current stories goes.
So, in the end, they only have a few months remaining to stay in Japan after their second year ends in the current graduation arc. Don't think that's a lot of time unless the author really stretches out some of the arcs after the graduation arc.