very bad idea, sivir is already strong as it is and her Spellshield is rewarded in the sense that it gives her back mana if she blocks a Skill. If you were to make her spellshield duration based instead of trigger-count based it would make her so undeniably op for the game that the weakest hotfix were to immediately reverse the buff. the reason why is simple. ADCs are decisive for the lategame where the teamfights begin, and teamfights usually do not last longer than maybe 5 seconds assuming both teams have a tank, an onhit champ, an assassin/mage, a standart support I.E. thresh and their respective adc. If the spellshield was to be duration based you could just trigger it once during the most crutial time in the teamfights, when people focus you, and you would negate more than half the damage dealt to you, every single hard cc available. roots, stuns, silences, supresstion, knock ups, knock backs. you would completely shut down entire champions or even whole comps with the press of a button while you lash out your crits and your superbly underestimated damage and AA resets.
So no, leave the spellshield as it is, because not only is it already how the mechanic itself works, it would prevent her from growing off the charts op.