Hi Tony,
Of course alot depends on how you use the trimmer, ie; do you spend alot of time with the line slapping hard, abrasive objects or do you go gently around them, but a quality line does make a huge difference.
I use Stihl brand line on my FS250 when I use the trimmer head. It`s black and I think it`s .130 diameter. It`s not very supple but it is very tough. An area that would literally take me twenty refills with Shakespear line can now be done twice with the same two pieces of the Stihl line. The Stihl line may seem expensive initially but I find it to be cheap when comparing it`s longevity to a few other brands that I`ve used. There are no doubt other excellent brands but this is what I`ve found.
How you store the line is also important. Keep it out of the sun. I`ve also heard normally reasonable people swear that soaking the line in water before use makes it last longer, although I can`t see the reason why.
Russ