Case: I'm currently in US on L2 (dependent on my wife's L1B). Wife's H1b got picked this time and she wish to postpone accepting the H1B because if she gets on to H1b right away, spouse( I in this case) will become ineligible to work ( on H4) until she has approved I-140 which is going to take 1+ years.

Question: How long can she safely keep the H1b lottery acceptance active without losing it ?

I don't have a specific answer here, but my understanding is that the usual idea is to choose "consular processing" for the H-1B visa. Perhaps your wife should talk with her company's immigration team about this.

That's right, selecting consular notification rather than change of status on the I-129 form can defer for a long while the start of H-1B status.