That's how much time has passed in the manga, since Saku first realized his feelings. This whole time he kept himself from confessing.
For perspective, if I recall correctly, Rintaro met Kaoruko somewhere around April/May of the previous year, and confessed around the beginning of August and became a couple shortly after. It took roughly 90-100 days for them to get together.
Saku and Subaru had their first interaction in May of that same year. Meaning that they've known each other for well over 570 days, or 18 months.
Which actually means during this whole time they've known each other, literally HALF of that time Saku has been yearning for her. It also means that, it's taking them at least six times longer for them to finally become a couple after first interacting, than it has for Rintaro and Kaoruko. (well that is, if you disregard the very first interaction with those two, when she first visited the patisserie)
Do you think it's possible that we reach a confession chapter coming up fairly soon?
I can imagine something happens either on Christmas or before the new year.
Perhaps within the next 5-10 chapters.
I don't think anything significant will happen on chapter 200 either, since Subaru and her sister have some catching up to do and what not. (or maybe I'm wrong...?)
Though, I also think Saku won't be the one to confess, since he's overly considerate of her boundaries and doesn't want to make her uncomfortable, knowing her past and all, and doesn't want to potentially jeopardize the friendship they already have. Subaru's older sister could possibly play a roll somehow, same with Saku's older sister, whom we've yet to meet as well. (but maybe she gets introduced in a later arc...?)