It really depends on the size of the plec. I cut a bit about an inch wide 3 times and put a spoon through them all to sink it. I usually do this twice a week but that's because my other fish will also eat it.
I used to give mine courgette which had a minutes cooking time in a cup of water in the microwave. This makes it sink without having to worry about tieing it down and is easier for them to eat!