As 6fingers said, it depends...
I worked as a guitar tech for years, traveling with a band that played 3 to 4 nights a week, anywhere from 2 hour to 6 hour gigs and the rule of thumb was change strings every 24 hours of play (approximately) any longer and the strings just sounded too dead to his ears. (He played mostly clean, Stratocasters through vintage Fender Princeton amps.)
Every recording session, new strings but since he wasn't playing under hot lights and sweating, we'd let the strings go until he wasn't happy with the sound, but never more than a week.
Me, for the past 'I don't know how many' years I've not played out, and only play a couple of hours a day, if that, so I let them go until they start to sound dead, unless I'm invited to jam somewhere, then I put on fresh strings.
The thing is, the strings losing tone over time is fairly gradual and most people don't notice how dead their strings are getting (especially novices) until they actually do change the strings and again hear that bright, punchy sound that fresh strings give you.
Some people like dead strings. I've known a few Death Metal players that say they like the dark, dead sound through their amps turned up somewhere around 23 with the gain clipping so hard it doesn't sound like a guitar any more. But at that point, you're actually barely hearing the strings anymore anyway, it's mostly amp.
So, to sum up this very long reply, it depends on you. If you like the sound of your strings after 8 months of play, go for it, if not, change the damned things.