Ineligibles are immediately eligible at D3 and will always come to campus. At D2, i've had about a 75% success rate with them showing up and at D1 I think its 90%+. If they don't show up, you get nothing. If they do show up, they are ineligible the entire first year. After that, they will have 3 years and get a 4th if they maintain above a 2.8.
Players that are initially marked as ineligible will continue to take the SAT through the end of RS2. If they get a higher score, they can become eligible. Their ability to improve their score is a function of their HS GPA. A very high level guide if you are looking at recruits the first day you can, 2.2 and less, id give it less than 25% chance they become eligible, 2.7 and up, its probably around a 75% chance they become eligible. For the GPAs in between, Its a sliding scale.
I personally love ineligibles. I think people gloss over them because they fear they wont show up or dont want to wait a season for them to become eligible. Because of this, I tend to get better players than I would normally be able to and there is a shot they just become eligible anyways.