If you've given up on the engine restart attempts QRH (7.16 in ours) had the dual engine flameout speed.
Quick estimate for a 737-800NG w/winglets -
Vref 15 +60 kts averages L/D +/- 2.5 kts.
Gross weight in lbs + 75 kts averages L/D +/- 4 kts.
Doesn't work at high altitudes, but with both engines out you won't be in the 30's for very long.