I fill the bags with a mixture of bought-in organic compost and my own home-made compost, which holds a lot more water. I find that the bags benefit from a daily water in hot weather, but otherwise you can go 2-3 days at least between watering, depending on how thirsty the crop is.
I've had great success with the following:
- capsicums (1 plant per bag)
- cucumbers (1 plant per bag)
- peas (4-5 plants per bag, with a wire frame and then a net tied to the balcony when they really went berserk)
Zucchini were a marginal success: they really need more space for their extensive root system and tend to suffer from blossom end rot. Pumpkins proved a waste of space and water: lovely healthy plant, but no fruit! Tomatoes didn't do as well as I hoped, but I used a vigorous, indeterminate variety (Tommy Toe); a bush variety such as Roma might do well.
Diggers advertise them as potato bags, but I've not had much success with that, after two attempts. The rampant top growth tries to climb out of the bag, pushing the sides down so you can't water properly, and the crop in both cases was miserable. Perhaps I just don't have the knack. These days I grow my spuds in the ground and have bucketloads of huge tasty spuds from an area 1 x 1.5 metres.
Several of my bags are in their third year and going strong. I'm fairly confident I won't need replacements for a year or two yet. But maybe I should buy a few more, just in case I find another corner to squeeze them into ...