I use Technifibre 18-gauge Biphase strings strung at 55 tension. They're pre-strung before packaging and the strings are expensive, about $24 a package. I play a firm, consistent game and play several days a week. The 18-gauge strings though break in the sweet spot pretty quickly, if I'm lucky the multifilament strings lasting about two weeks. (Still, considering my harder hitting play and my frequency of play, I consider the strings surprisingly durable. I don't though apply excessive top spin on baseline shots, but the 18 gauge provides a little more spin than lesser gauges.)
Yet I figure by the time the strings break, it's time to replace them to maintain liveliness and my preferred string tension. (I string my own rackets using a top-dog stringing machine, and the strings in my play have never broken near the racket frame. I'm particular and I buy the strings by the coil reducing the string cost to a little cheaper. I carry four of the same rackets but I use an alternating restringing method and an alternating racket-use method. In other words, I don't have a bunch of rackets just sitting in my bag with strings losing tension.)