1 Answers
AWS provides 99.9% Availability for S3 Intelligent-Tiering according the to user guide (see Comparing the Amazon S3 storage classes here).
Since S3 Intelligent Tiering is either storing your file in S3 Standard (Availability 99.99%), S3 Standard-IA (Availability 99.9%) or S3 Glacier (99.99%) I would assume the availability would be the lowest of the tiers. It looks like there are some new optional features for using Glacier Flexible and Deep Retrieval which would make you wait from minutes up to to 12 hours.
Look at official docs in https://aws.amazon.com/s3/storage-classes/ . 99.9 is correct