This is only a partial answer, because I'm currently researching the same question.
Whatever material you use, it will need to pull water up from the bucket before it conducts it down to the plant. And if you fill the bucket before you go on holiday for a month, then as the water level descends the capillary will have to pull water upwards through an increasingly long distance.
One possibility is to use a sink, either above the plants or below it, with the capillary material connecting the water to the plant, and just leave a tap dripping.
The water level in the sink will then stay at the same height, and you will know that water will always have to be pulled upwards through that fixed distance. Of course you may not wish to do this if your water supply is metered. It could also be argued that it is antisocial because it's a waste of water.
Then again you can possibly get around both of these objections if you find out how much water drips into the sink per hour when the tap is turned to a certain exact position, and how much water leaves the sink up the capillary material every hour on average. There will be likely to much more fluctuation in the latter than the former. Assuming you equalise these two rates, you would need to make sure the rate you get is reasonable for your plants and for the surface they're on.