For most people, a fast of eight to ten hours should suffice to get an accurate triglyceride level. Of course, there will be the occasional person, starting off with massively elevated levels, who may take much longer to get the TG’s down.
As the paper cited above suggests, though, there may be some misleading readings obtained after a prolonged fast if it included strict salt and water restriction. Simply put, if you dry out during a fast, your lipids become more concentrated.
Truth Seeker - Yes, not fasting will lead to higher triglyceride levels. And, yes, you would be generally right to say that if a non-fasting cholesterol is normal, the fasting cholesterol level will also be normal.
The reason I say you are only “generally right” about nonfasting cholesterol tests has to do with HDL and LDL. Here is an article and here is another that show that generally there’s not much difference for cholesterol measured in the non-fasting state (unlike that for triglycerides) but there can sometimes be some significant differences (and in directions you might not expect).