The exact amount of time it will take to download iOS 18 mainly depends on the speed of your internet, as well as the number of other iPhone users currently downloading the update. It can be pretty slow at launch, but the process speeds up over the first few hours as keen fans download and install the update. As you won’t be able to use your iPhone while the update is installing, if you’re concerned about it taking too long during the day then it’s advisable to schedule the download for overnight.