Can a student-athlete enter the portal a day, a week, a month or whatever after signing an LOI?  How does an LOI bind a prospective player?

A "Letter of Intent" is binding, however there are many examples of where a school has let an individual out of their LOI.

 

I'm not a legal scholar, and I didn't stay at a Holiday Inn recently😂

I don't know exactly... but there are some interesting examples where students transfer pretty quickly after signing. 

 

I believe (and I may be wrong) but a student athlete after signing a LoI can ask the school to be released for no real consequences but they can't use the transfer portal at that time. I believe a student athlete must actually enroll with a school before they can transfer.