Generally speaking, strings become less bright with time. A key factor in this is the accumulation of dirt, sweat and other grime in the windings of the string. Flatwounds start of relatively mellow but last for ages - roundwounds start off much brighter but have so much more room for all the grunge to hide
The muck can largely be cleaned off with various techniques (eg boiling or soaking in various solutions). However, there is also physical damage to the strings - for example, the frets start to wear down the windings or the windings come loose from the inner core. These problems tend to result more in difficulties with intonation rather then lack of brightness per se and can't just be cleaned away.
Now, as to how long before changing strings, it depends on a range of factors. Assuming your strings are physically undamaged then it comes down to how much zing you like with your string and, if you live for the sound of fresh roundwounds, how much you can afford.
If you do decide its time for a new set, how about recording some stuff with the current set, recording again with the new set and then again after they've had a couple of weeks to settle down - that will give you an audible reference on how strings change over time.
Wulf