As ballpark figures:
Inverted at 400 feet, no descent rate,and the seat will save you.
Ejections at up to 60 degrees of bank aren't too much of a problem (assuming this doesn't stick you straight into the valley wall, which has happened) - you still have half the seat rocket thrust upwards, but it gets rapidly worse with just a bit more bank.
Altitude = at least 1/10 rate of descent (units per minute) and the seat will save you. So, supersonic straight down (e.g. post hole manoeuvre) and you must be out by 6,000 feet at the latest, preferably 10,000 feet. And that's AGL of course - gets scary in the mountains.
Rate of descent builds up rapidly and has an increasing effect on survivability at very low altitudes. The advice on the Harrier was Red Caption in the Hover - Eject. By the time one had worked out that there was an engine problem, it could have been too late to eject.