I recently gave a (in) progress grade on a ceramic sculptural assignment which is going to take about 3 weeks total for the constrcution and what I did was mention that we were now about 2/3rds of the way through, reviewed progress according to a daily timeline suggesting how far along they might be at that point in time and then reviewed the important aspects of this assignment (proportion, expression, what had been 'blocked in' at thsi point). I graded them on just these aspects-some had done quite a bit of stalling and not progressing along and their grade reflects that currently. This is HS.
Hope this is useful,
Jude (who won't have a spring break for a while yet!)