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It takes 2 days and a half or 60 hours to refill voting power to 100% from 50%. Since voting power takes 24 hours to refill for 20%, it would take 2½ times 24 hours (2,5 x 24 = 60 hours) to refill for a total of 50% (50% plus 50% equals 100%).

I believe voting power regenerates at a steady rate of 20% a day. For example, to regenerate from 0% to 100% takes 5 full days. Therefore, if you start with 50% voting power, it should take you 2.5 days to regenerate to 100% voting power.