i used acrylic felt fabric as a wicking mat - you can get it around xmas time in green. or most other colors. it does not mildew (as cotton would) and is thicker then flannel and easily washed. it wicks very well.
i often set up a few smaller young plants in a plastic shoe storage box and use upside down smaller plastic box of some kind as a prop. depending on how much water you need in the tray, you can use squarish 'hot-food' take-out container bottoms too, they are quite sturdy. for heavier plants i might fill the large plastic box with lots of upside-down sturdy 3-4-6" plastic pots and then you can put egg-crate on top to create a continuous surface. i put all smaller plants on water-wicks that i then drop thru the grid into water-tray- so i don't have to monitor them as much.
i use double ply acrylic yarn as a light wick, doubled for more wicking and thick braided nylon rope with poly-core for larger pots, even doubled for thirsty plants. the wick goes inside the pot, around the bottom and then up the side to the top to increase wicking. it does serve as 'wick-out' for perched water and wick-in for watering. but your soil mix needs to be adjusted to wick well - it needs at least 30% peat and 30-50% perlite.
also the higher you pot is, the faster the top soil surface will dry up - mulching with something helps a lot. it could even be loose shredded paper. but i do water from top too for this resaon - to rewet the top part of mix well and flush the mix from time to time.
i had no problems with salt build-up ever or root-rot. but i do let them dry up somewhat from time to time too, when they absorb all the water.
but in general i'd say - you can keep the wicking going for 4 weeks easily. i do even more - great for vacations and busy time!
