Period panties are designed to be worn day-long, just like regular underwear. If you have selected the appropriate absorbency level for your period flow, this should be totally feasible. However, if you have selected a lower absorbency level than your period flow on any given day, you may have to change underwear (or use in conjunction with a tampon or pad).
To recap the absorbency levels:
Super Absorbency
These period panties are a total replacement for disposable period products - absorbs approximately 8 tsp, 40 ml, or the equivalent of 8 regular tampons worth of liquid.
High Absorbency
Ready to stop leaks whenever - absorbs approximately 4-6 tsp, 20-30 ml, or the equivalent of 4-6 tampons worth of liquid.
Medium Absorbency
Confident backup protection for heaviest days - absorbs approximately 3 tsp, 15 ml, or the equivalent amount of 2 tampons of liquid.
Light Absorbency
Panties that offer protection for your light flow days - absorbs approximately 1 tsp, 5 ml, or the equivalent of 1 tampons worth of liquid.