DD does art, a coursework-heavy DT subject and 10 hours of extracurricular dancve per week. Yes, Art takes longer than her other homework BUT she is utterly disciplined about it - X needs to be done by time Y, mounted, annotated, done.
Sort her time management. Sort out a daily timetable - an hour of art, then an hour of other things. Also encourage her to swap to something else as soon as she is not producing art - so get as far as she can, stop, do something else, come back to it. DD used to spend time staring at her art work in despair - now she goes and does 30 mins of something else, and comes back again to her art if needed.
Also get everything together, take it to her art teacher and, if needed with you, go through and decide what MUST be done, what COULD be done, and what ISN'T going to be done and it's fine. DD has swapped several pieces beteen e.g, work she did over the summer and preparation for her GCSE mock - not what they were 'intended' for, buuit she has too much in some areas and not enough in another, so a little readjusting worked well and meant that art she had done wasn't wasted, and time was saved..