How does the USCIS 90-day rule work for EB-5?

I always advise clients to wait at least 90 days before filing adjustment of status if they entered the U.S. on a non-immigrant visa that requires strict non-immigrant intent. The main issue is whether you entered the U.S. in B-2 status with the intention of filing for adjustment of status. This is a subjective issue and very fact specific. It is certainly possible for facts and circumstances to change after admission to the U.S. - that being said, waiting a few months prior to filing AOS is advisable.

The issue is when you entered, did you have an intent to apply for adjustment. The 90-day rule is merely a presumption so if you entered with a plan to adjust, it can be a problem. But if you entered without a plan to adjust, and circumstances changed after your entry, then you are now eligible to apply to adjust status in the U.S. After 90 days the presumption acts in your favor but need to look at the actual facts - presumptions are merely evidentiary burdens, and one must also examine the facts of each case.

I think you can argue a change in circumstances and file when you want as you were not expecting the I-526 approval on this trip.

If you are in the US on a non-immigrant visa (e.g. B-1), too risky to apply before 90 days because of presumption of immigrant intent.

There is no requirement to wait 90 days, but there is a requirement that the visa be approved and also available. If the visa is available and you are in the U.S. in legal status, you can file to adjust.