It's hard to determine the minimum connection time without a few more details. For the sake of argument, I'm going to assume you're flying JNB-EWR on UA187, and that you're asking this because you are considering booking two separate tickets. Also for the sake of argument, I'm assuming that you're then planning on doing DL4471 to MSP and then connecting onward to BOI.
The standard connection time between different international to domestic airlines at EWR is 1.3 hours, and for UA in particular it's 2.3 hours. These are the minimum times that would be required to put you on the same ticket.
Consider that a flight boarding at 7:55AM is going to try to push back at around 7:25AM, and may release your seats before that if you're not at the gate. Also consider that UA187 is only on-time around 77% of the time, with an average of delay of 18 minutes and a maximum delay of up to 1h 29 minutes. That might leave as little as 45 minutes optimistically to deboard, clear customs, clear security and board the next flight. You'd miss the flight completely in the pessimistic case.
It looks like as of right now, that flight arrives into Terminal B, which is the same terminal that Delta operates out of in Newark, so there's no Airtrain/terminal change involved, and you can try to reserve a spot to get through security faster with the Newark VirtuaLine program. (Again, assuming you're trying to connect on a Delta MSP flight)
It's probably possible, but I wouldn't book it in your situation. I have global entry and am pretty familiar with that airport, but it's pretty tight. I'd leave at least an hour and a half (if not more) for that. Ideally you want to book this on a single ticket so that UA is on the hook to get you to BOI, even if there is a misconnect.
Assuming that you decide to book this anyway and don't make it, Delta may accommodate you under the Flat Tire rule, but it's really up to the agent. I'd be prepared to pay the full cost of the onward fare (possibly minus what you paid for that ticket) if you are unsuccessful.