Edit: Before you read the following; remember how much these cost. For the money they are fantastic ( ~ £6 ) I used these to hold up my christmas lights on the outside of our house as there was no other way unless I used a lot of putty or tape to do the job.
I found that most of them stuck really really great but that just a few managed not to stick well at all. This may have been the surface but meant that I managed to use all the spare sticky tabs except 2.
Keep in mind that I didn't get to clean the surface much and this was on the outside of a dusty house sticking onto plastic surfaces.
The ones that did stick are holding up our thin wire set of 600 icicle lights and thicker wire set of 400 coloured lights. They had difficulty in places with the weight of the thicker wire and I had to use them about 10 to 15cm apart to hold; but for the mostpart they were fine.
Two packs of these held the 600 icicle and 400 colour lights; 9.5m of icicles and 20m of coloured lights; so I'd say one pack per 15m at least.
They come off fantastically easily if you need to remove them; follow instructions if you have them on a painted wall. You pull parallel to the wall, not perpendicular. This will release them with no marks or pulling off of paint.
I will be buying these again.
They are small though; the 5 wires of our coloured lights only just fit inside the clip. they are not for thick AC wires for example. They are about 1.5cm squared in size