With an I7-7700K you would get a 50% boost compared to what you have today. To effectively halve the build time you would need a 100% boost plus it rarely scales 1:1 since the motherboard RAM speed and disk drives could also slow down the compile times.
So lets say your light build with your I5 takes 30 minutes I’d guess the I7 would do it in 23-25 minutes.
You should save your money and buy a new PC with a Ryzen 3800X which is 300% faster than your current I5. If you get a fast SSD drive and some fast RAM your 30 minute compile times could be done in less than 10 minutes.