When one or more of the following happens to the strings, it is time to change:
1) becomes physically damaged or breaks, including accumulation of fret dents to the point it causes noticable flat marks on the windings
2) won't hold tune
3) intonation problems: some fretted notes fret fine, others don't, and the bridge saddle is set up correctly
4) wobbly tone, or overtones won't lock
5) noticable corrosion
6) tone becomes too thumpy or loses sustain more than desireable or compared to the other strings on the bass.
If you like the Fender strings, there is no reason to change unless you are curious, want a tonal or feel change, or the balance of either tone or feel string-to-string just isn't happening for you. For example, until I went to tapes, I always kept and extra E string, because my perspiration eats nickel, and for me the E string, irrespective of brand, model or gauge of string, would always die first, sometimes to the point that I would put a new set of strings on for a Friday-Saturday two-night gig, and replace the E string for Saturday.
Second, keep the most recent set of strings you take off the bass for the following reasons:
1) to know how long to cut the leader so it is long enough so the string has enough wraps around the tuner post to be stable, but not so long that it binds on the bushing or overlaps.
2) if a string breaks in a gig, putting the older string back on will usually be a better match, because it doesn't sound as bad for the bass to sound "tum, tum, thud, tum" from the old string to get through a gig than it does to sound "tum, tum, twang, tum" from the new string, and the old string will still hold pitch better than a new string going flat in the middle of a song from going through its initial stretch-out phase.