There is no universal number of burn-in hours that makes every IEM better. You can start listening normally. A claim that an earphone needs 50 or 100 hours should come with evidence for that model.
What would a real improvement require?
Even if a mechanical property changes with use, that does not establish an audible improvement. To show a meaningful change, measurements need repeatable fit, the same input level and enough repeat tests to separate it from measurement variation.
In an interview published by Brüel & Kjær, Shure engineer Yuri Shulman rejects burn-in as a general requirement for Shure headphones. That is a manufacturer’s position, not proof about every IEM ever made. Read the original engineering interview.
Why your impression can change
You may find a better ear-tip seal, insert the earphone differently or become familiar with its tuning. Different music and volume levels also change the impression. Hearing a difference later does not, on its own, identify the driver as the cause.
What should you do with a new pair?
Find comfortable tips, check both channels and listen at your usual moderate level. Use a few familiar tracks. If the sound remains unpleasant, consider the fit and tuning while you are still within the return period.
Do not use days of loud noise to cure a faulty or uncomfortable IEM. Persistent rattling, channel imbalance or distortion deserves troubleshooting or support, not a longer countdown.
Related: IEM problems and fixes and When an earphone upgrade actually makes sense.