There are huge variations in the time being spent on the baseline assessments, with some teachers devoting more than 52 hours to a class of 21 pupils, while others spend less than 10 hours.
However, the union's "Uncommon Entrance" survey also reveals widespread acceptance of the principle of baseline assessment among its sample of 92 English schools.
A total of 65 per cent of teachers agreed or strongly agreed that it helped them plan teaching to meet the needs of pupils.
Doug McAvoy, NUT general secretary, said: "The important message that comes over is that teachers don't see anything wrong with the baseline assessment scheme and they feel comfortable with it."
However, the "enormous" variety in the time required by the various schemes being used by teachers was a concern.
Mr McAvoy called for the Qualifications and Curriculum Authority to introduce stiffer time restrictions on schools.
National guidelines say a maximum of 20 minutes' teaching time per child should be devoted to each assessment outside normal classroom activities, but the union survey found that teachers were actually spending an average of 29 minutes.