I suppose most people would consider the "standard" lineweight to be 0.25mm. This reads well in most cases and is the standard ISO lineweight. For emphasis, you could use 0.35mm and for heavy lines 0.5mm. A slightly lighter weight than the standard would be 0.18mm.
Traditionally (working on a drawing board), those are the 4 pens that most draftspeople would use as standard. Of course, AutoCAD offers a greater range and more options than this but I reckon just about any drawing can be completed with a combination of .18, .25, .35 and .5 (sometimes a .7 is useful for heavy borders etc.).