There are too many variables to hazard a guess at flight time. Someone may guess on a percentage less flight time than on a 'normal' flight. But it would still be a guess.
I could depend on how charged the packs are, how long they have stood for, how cold the packs are, how much additional power is used, etc etc.
What's the twin ?, if it wont glide, (it won't really fly dead stick or power reduced), I can only assume it has a lot of drag, or horrendous wing loading.
I fly twins, (and a fours), and as there is always that risk of one motor cutting before the other I get a feel for when the motors are staring to loose their edge as the volts start to drop. Then it's gentle flying and start the landing circuit.
During the winter, try to keep Lipos warm, something like an insulated box or bag. Lipos loose their edge quicker if flown cold.