How long it will take depends on a lot of factors. Most likely S3 will happily receive your upload at whatever speed your server can send it, minus some overheads. On a 1Gbit link that’s probably going to be ~5 minutes.
Rebaking and resulting jobs are likely where the vast majority of time will be spent. How long this will take could vary wildly by how many posts your instance has, how much and what kind of content posts typically have and the clock speed of your server’s CPU. How long the resulting jobs take will somewhat depend on the same things as well as how many CPU cores are available.
My understanding (someone please correct me if wrong) of the migrate to S3 process and the rebaking is that the existing uploads won’t be removed until some point after they have been disassociated with the post (after the rebake has done that post) so the instance can continue being used without missing uploads while the migrate and rebake are happening.
If you’re on a virtual server with only one core available, it might be worth considering upgrading to more cores for the rebake and then downgrading after it’s complete. The process will take significantly longer on a single core and Discourse performance may be severely affected during it.