Just talk to her, brush her softly and slowly, patting her to rewarding her every time she relaxes. If she's got mud caked on her that could cause pain when grooming maybe use a wet towel to soften it up before your curry it. You don't want her to associate pain with grooming. She's probably never had anyone show her love in this kind of way. Personally, I think grooming is crucial in training and gaining trust. It's a happy time between horse and owner.
As far as time sessions, you've gotta work with her and adapt to what she can handle. You could start off with short sessions, just going until she's calm and then add on 5 minutes or so from then on.