When installed properly the ice and water shield should last as long as the shingle roof which is around 20 to 25 years.
The ice and water shield typically is installed only along the overhangs and in the valleys of the roof and can be left exposed to the elements for 30 days.
As long as you get the ice and water shield covered with shingles within 30 days the ice and water shield should easily last the 20 to 25 years that the roofing shingles should last.