Talk also with the school counselor. He or she may be able to meet with you and the teacher to try to work out a solution to this problem. The counselor may also be able to give your daughter some individual time to help her feel better about her frustration with the work.
If things do not improve and your daughter is staying up late to finish her homework and crying every night over it, talk with your pediatrician. This may be seen as an issue that could affect her health. Ask your doctor to write a note to the school stating that your daughter is limited to 45 minutes (or an hour) of homework each night because of health reasons.