Does anyone have an estimate as to how long does it take for promotion cut-off score to come down from 798 to anything attainable ?

Completely dependent on your MOS. As a rule, the more technical the MOS, the smaller the field, the more you need E4s and the less you need supervisors and NCOs. For instance, if you look at chaplain assistants, there is one E7 for and entire Division.
You promote when a spot opens. A spot opens when some leaves the Army or promotes out of their slot. So a small field of higher rank typically translates into slower promotions for the lower ranks. (this is not true for officers, which is why so many MI officers are branch detailed to Infantry as LTs).
Your best bet is to look at the history of your MOS promotion points. If it has been maexed for a year or more, it's time to find a new job.

It entirely depends on availability of open billets for your next higher grade. That can fluctuate a lot depending on your MOS strength, retention, and promotion rate of your seniors to the next higher grade... The only thing you can really do is speak to your representative at the Office of the Chief of Military Intelligence (OCMI) for current projections (still highly speculative). In the mean time, do everything in your power to increase your promotion points so that you're near the top of the list when promotion points do drop.

try flippin years!