forever, or until rust version update (might be often for nightly builds), or until build flag change or cargo clean, whichever happens first. It's also only a per-project target directory (not across all projects you compile).
Even for people on nightly builds of the compiler, I would be quite surprised if nearly anyone actually bothered to upgrade their compiler every night. Most people on nightly are just going to be there for the use of a specific unstable feature.
