I personally used the Convection Bake option on my range for the first time this year (2012). It was a 21 lb. turkey. I did the first step placing the turkey in an uncovered roasting pan in the oven preheated to 475° for 20 minutes.
I then reduced the temperature to 250° and left the turkey in the oven uncovered for about 15 minutes. I then opened the oven, quickly placed my electronic thermometer in the thickest part of the breast, put the cover on the roasting pan and closed the oven.
In a conventional oven the additional cook time would have been 21 x 20 lb. = 420 minutes or 7 hr. But the breast reached 170° in just 4.5 hr. which is 36% faster! I then checked the temperature of the thigh and it was a perfect 180°. I put the covered roasting pan on the stove warming area to keep it warm. I didn't worry about it drying out since it was sealed from the first step. Everything went perfectly.
Most people who have a convection oven report that their turkey got done faster and my results support those results. My one experience can't be used as a rule of thumb. Some turkeys just get done faster than others. But I think you can assume that it will get done significantly faster. Maybe you want to plan 20 minutes x 67% = 13 minutes /lb. for the second phase but leave yourself enough leeway to put it back in if it's not done. Or you can play it safe and use 20 minutes and... if it gets done quicker... do what I did to keep it warm.
I hope this is helpful.