Likewise Waltz issued his resignation and his seat will be filled on the same timeline. Waltz will officially resign on January 20th.
Waltz' position in the Administration doesn't need Senate confirmation.
For Stefanik, she still hasn't resigned her seat. Under New York state law, Hochul will have 10 days from the resignation to call a special election, and the special election will have to occur 70-80 days after that.
If Republicans are smart, they'll hold off on confirming Stefanik till April.
Once the special elections occur, the House can seat their replacements quickly.
So Republicans will have at least two vacancies from late January till April. Maybe three.
They are on track for a 220 seat majority. With two vacancies they'll have only 1 seat to spare. With three vacancies they'll have 0 seats to spare.