1. I'm pretty sure it's forever (unless deleted), but you have the option of
resetting the "summary rating":
Individual ratings inform your app’s summary rating, which is displayed on your product page and in search results. This summary rating is specific to each territory on the App Store and you can reset it when you release a new version of your app. However, we recommend using this feature sparingly; while resetting the summary rating can ensure that it reflects the most current version of your app — useful if an update addresses users’ previous concerns — having few ratings may discourage potential users from downloading your app.
The "summary rating" is the 5 stars shown next to your app's title on the App Store.
2. People can only leave reviews once (It's tied to your Apple ID).
To delete reviews, you can flag inappropriate ones for Apple to remove.
Users can also delete their own reviews.
