. I came up with three options. There are probably a bunch more out there.
Jho sy a té avoî k' î nevése a la Sen Majheré.
[ʒo.sjaˈte ʔaˈvuː kiː.nəˈve.sə ʔa.la.sɛ̃m.mɑˈʒʁe]
1S.NOM be.1S to 2S.DSJ until SBRD 3S.NOM snow-IPF_SBJV-3S at DEF-F.C.ACC saint.F.N.OBL Margaret.OBL
I'm yours until it snows on Saint Margaret's Day.
Margaret of Antioch is celebrated on July 20, so snow is the last thing in the Silvish forecast at that point. This option is probably the most positive or romantic. It's evocative, and it invokes a saint associated with childbirth and pregnant women, which could connect to loving relationships of a few different kinds.
Jho sy a té avoî k' la ppoela eusön de den.
[ʒo.sjaˈte ʔaˈvuː klɑˈpu.la ˈʔø.sŋ̩ dəˈdɛ̃ŋ]
1S.NOM be.1S to 2S.DSJ until SBRD DEF-F.C PL-chicken have.IPF_SBJV-3P INDF.PL tooth
I'm yours until chickens have teeth.
I borrowed this option from French, and it strikes me as kind of wacky in this context. Is it a promise or a threat? Who knows?
Jho sy a té avoî ke tu m' atrapése un daū.
[ʒo.sjaˈte ʔaˈvuː ke.ty.ma.tʁaˈpe.sə ʔœ̃n.daˈy]
1S.NOM be.1S to 2S.DSJ until SBRD 2S-NOM 1S.DAT catch-IPF_SBJV-2S INDF.M.C dahu
I'm yours until you catch me a dahu.
Finally, this option, which I'd say gives the most negative vibe. The dahu
is an imaginary creature that inhabits mountainous areas in Europe. So this option works best as an impossible quid pro quo (e.g. Jho t' edyeré kan ke tu m' atrapése un daū
"I'll help you when you catch me a dahu"). In this translation, the speaker is saying they won't leave unless the addressee does something impossible. Really puts the onus on the addressee.
On a general note, Silvish gets to use its imperfect subjunctive (a counterfactual) in the "until" clauses, adding extra oomph to the "This will never, ever happen" idea. The other possible verb form in those clauses, the present subjunctive, is a better choice for things that are certain or likely to happen.