I stop when either I get tired of what I am working on or I start to hurt. I have this one spot on my left shoulder/neck area that starts and if I don't stop I will be out for days. and taking meds I don't like. I took yesterday off because my little fuzzy face was sick and wanted me near so I spent the day reading instead of sewing. I guess I needed the break to since I did miss the machine one bit.
As far as the fabric look at maybe piecing your border, sometimes cut up in smaller pieces it looks better. or as Doghousemom suggested trade with someone.