Pretty dependent on lots of things. Are we assuming both engines conk out simultaneously?
I was given a leaflet suggesting procedure for double engine failure on a B737 a while ago. It gave a circling visual approach, similar to the old "high key" / "low key" thing, and a glide approach clean one dot above an ILS glideslope.
The danger with turnbacks - as it is with light aircraft - is that you run the risk of stalling and spinning, or not getting where you want to. I suppose if the alternative is a mountain in front of you, you might give it ago. I'd like about 4000' below me to turn the aircraft through 180' - but that wouldn't get me back to the runway.
You ought to be able to work out what gradient you would need to climb out at to allow you to have enough height to turn around and land back on the reciprocal. My suspicion is that it would be something considerably more than you are likely to have until about 7 or 8 thousand feet.