Hi,
There is no hard and fast rule about how often you need to change the strings.
The general rule of thumb is that the strings need changing when they are showing signs of rust.
Rusty strings not only
sound bad, they also
feel bad. Imagine doing some slides on those thin, rusty strings. Not a nice thought eh?
There are also many factors to consider, such as if you're using coated or uncoated strings.
Coated strings like the
Elixirs,
Cleartones,
D'addario EXPs etc, have a special polymer coating that resist rusting/corrosion much better than the normal un-coated strings. They are a bit more costly as well.
And then there's also the factor of whether you wipe down your strings every night and apply some sort of protectant on it, products such as
String Life,
Fast-Fret, etc. Which are readily available in most guitar shops.
For my case, I play daily, and I wipe down my strings every night and apply Fast-Fret. When I do use coated strings, they can last for about 3months without seeing any signs of rust. But I usually change the strings sooner than the allotted 3 months because I simply can't stand it when it sounds dead.
With normal strings, they can last about a month or more.
I know some people who change after each performance, or change the strings weekly; as soon as their strings lose their 'ring'.
As demonstrated, there
really is
no hard and fast rule. Don't bother what people say regarding how often you need to change the strings. You yourself be the judge.