I had to do this once. Blown alternator, dead battery. Luckily had a backup battery and it got me home 10 miles away.

I had the alternator fail on me while driving, though with a working battery.

It was at night and I made it about 70 miles, mostly on the interstate. The headlights were noticeably weaker by the end.

That was in line with the estimate of 3 hours at the end of this piece.