This isn't a snarky answer, but take every champ that has less than 20 lines in their VO pool and divide that by two. That's the number of years it will take to get every champ's VO up to snuff.
Explanation: ChampUp only has two teams that would be working on a new VO list for reworked champs and they each only get through one rework a year (Ryze being an exception). If Riot added a third team to ChampUp, bumped up the cadence schedule or created a crew that worked entirely on new voice-overs, that number would be reduced.
Still, you're looking at not only champs that need reworks but champs that got visual upgrades in 2014 and 2015 that were skipped over for new VO lines. That leads up to a lot of years.