When you start a new account Tracker, it will backfill with 30 days of historical data in a matter of seconds. From that point on, it will track new posts in real time moving forward. Follower counts begin tracking from the day your Tracker is created, and are not backfilled.
When you start a new hashtag Tracker, it will backfill with data on up to 1,000 recent posts within the past 30 days, whichever comes first. After that, it will track all new posts in real time.
Please note that historical data may not be full-fidelity due to limits from the Instagram API. Real-time data will be complete, so if you're looking for full coverage, start tracking now!