Real sticky subject this mate
My first set lasted about six months of high use use ( twice or three times a week or about 6 hrs a week)
They then became a bit soft so I bought some new ones and they lasted less time due to me doing a lot more hours
Having said that my first set of springs are still useable if I don't mind bottoming them
I know people who have had a set of springs for well over a year and they are still going strong so it really does depend on your technique and how hard you bock (and how fit you are)
But most people get 6 months + out of their first set of springs and at 70 ish quid a set that is cheap fun
Jason