In Vetus Testamentum, Vol. 12, Fasc. 4 (Oct., 1962), pp. 500-501 Reuven Yaron's claims כעת חיה in Genesis 18:10, 14; 2 Kings 4:16, 17 means "at this time, next year" and this seems to check out when consulting the modern versions. He also claims that the Akkadian expression "to life" ana balat is used in the sense of "next year". Yaron then suggests that this could hold true for Hebrew with לחי which we see in 1 Samuel 25:6.
Many English versions get a little slippery with their treatment of כה לחי. Some translate it as "that liveth"(KJV, ASV) or as wishes for "long life"(Darby, HCSB, Lexham, NASB, NIV). RSV, ESV, NRSV seem to treat it as modifying ואמרתם to a greeting or salute. NABRE changes לחי to לאחי (my brother). NJPS translates it simply "[Say] as follows: To life!..." So this is a phrase that there has been significantly different attempts to translate.
The New English Bible (and REB), "All good wishes for the year ahead!" Robert Alter's "The David Story" Renders it "Thus may it be this time next year..." These two renderings follow Yaron's line of thought.
In Holladay's Concise HALOT, the 2nd listed definition for כה is - "2. temporal, now: ad-koh until now Ex 7:16, wead koh meanwhile 1K 18:45;..."
If Yaron, NEB, REB, and Alter are right that לחי is temporal, could כה לחי mean something like "now to next year"?