Do you mean stroke weight? If so, I wouldn't go below .25 pt if you can avoid it.
FWIW, It's always a safe idea to contact your printer, if you know who's doing the job. Depending on the plate-making equipment, etc., some printers can hold smaller rule widths than others. Paper stock is also a very big factor; you would get a good, solid line on a coated stock vs. uncoated, but a good smooth or super smooth brand of uncoated will also hold well. On the other hand, if your stock is textured (i.e., linen finish, laid finish, etc.), you will have difficulty holding thinner lines. Hope this helps!
IIt all depends on the quality of print and type of paper stock Matt, silk, gloss etc.
i the job can afford it and the printer is good I would go for a wet proof.
I Agree with Peter anything below 0.25 is pushing things, but a good wet proof will prove invaluable.
best of luck I hope this helps.
At 150 lpi a 3-5% halftone dot output at 2400 dpi is around .00166" (.12 pt). I would think most presses could print that line, but it would be barely visible so wouldn't function well as a graphic rule.
Are you worried about the readability of graphic rules or the print-ability of fine line art?