How long should a string last??  Is three thousand shots about right before changing strings?
Today I was shooting and had a catastrophic string failure.  The lower loop broke.  I am shooting a 55lb Bear Super Kodiak.  With my thirty inch draw the poundage is a little higher.
String was continious loop, 14 strand dacron that I spun myself.  Looking at my notes I figure it had 3300 shots on it.
The irony of the whole thing is I was trying to shoot with a release that someone had left me, it was the first shot and I wasn't quite to full draw when it let loose.  I'm guessing the Trad Archery Deity wasn't happy with the release gimmick.

Sarge, I to shoot B50, I would think it should go more than that. I shoot 20-30 arrows 5 days a week which works out to about 5,000 a year and never have had a failure. I change mine if they start looking bad or yearly. I'm shooting a 12 strand endless string and wax it every 4-6 weeks.

The key is to check your string and keep it waxed as mentioned.  I replace mine at the first sign of wear on the string.  They are pretty cheap and I have not broken one yet.

i change my strings about every other year.
i shoot dacron and like a little stretch still in them.

I shoot a 14 stran B50 string on a 45lb longbow, have never changed the string in over a year...........James