Cartos that you fill up just to have ready to vape when you need them should last as long as the
juice in the bottle ... about 2 years. Cap them using the same caps you removed to fill them.
If you are swapping cartos and
vaping one much later, I can't tell you much, other than to use them up in months, not years. I guess it's possible for
juice to change once it has been vaped in a carto. I always find dark crud in a carto that's been used exclusively for a week. I sometimes snap the end off, pull out the coil, and then remove the filler and unroll it. The burned looking residue reveals why it starts to taste bad. I just do that to determine how much crud occurs in a period of time.
Lots of people label cartos with the juice they filled them with and rotate them on a whim. I've never heard of ill effects from doing so. If a carto lasts a week and you rotate a dozen, then it stands to reason that those will be in service for many months.