3,464. The Leniency Is Only for Shabbos

Hilchos Maaser 12:2

Even when one can rely on an unlearned person to eat demai on Shabbos, he may not eat that produce after Shabbos until he takes the necessary tithes for the whole volume, including both what he ate on Shabbos and what’s left. This is because the leniency of relying on the unlearned person was only made for the needs of that Shabbos. If Shabbos and yom tov fall back-to-back and someone asked an unlearned person about produce on one of them, he may eat it on the other because there’s no opportunity to tithe the produce in between. The same is true regarding the two days of yom tov outside of Israel.

Hilchos Maaser 12:3

Let’s say that someone takes an oath that his friend should be his Shabbos guest, but the friend doesn’t trust the host regarding tithes. In such a case, he may ask the host about it and eat based on his words that first Shabbos. The next Shabbos this happens, however, even if the host takes an oath not to benefit from his friend unless the joins him as a guest, he may not eat without tithing the produce as one does for demai.