As long as possible.

Generally speaking, the longer you go between changes, the more you're going to want to layer up. Flannel on top and PUL at the back are your best options for keeping a pad thin and making sure that almost every fiber is saturated before it overflows, but you don't want to wear a PUL pad to the point of saturation because when it finally does overflow, you will have a messy flood on your hands. I think a pad would get uncomfortable before it got that saturated anyway.

Recommended Pieces: Any.
Recommended Materials: Wool, PUL or synthetic fleece.

I change pads frequently.

Silk and flat cotton tops have a nice fresh feeling for short-term use. If you're changing frequently, you probably won't need to worry about waterproofing and you won't need to layer up as much.

Recommended Sampler Sets: Any of the all natural or all cotton/hemp options. Short/light or long/light sampler set.

It depends on where I am or how heavy my period is.

The amount of time you expect to pass before you get to the next restroom will help determine the number of layers you wear at a time. If you're out and about in the world, pack some extra pad pieces in a small wet bag, zippered makeup bag, or even a re-purposed pencil pouch. The more breathable your carrier is, the less likely your pad is to develop odors.