You either cook it for a really, really short amount of time (seconds or a couple of minutes at the most) or for a long time (an hour or so). the trouble comes when it's 5 minutes or 10 which, in restaurants, it mostly is.
This isn't a "real" recipe but it's got Spanish flavours. The idea comes from a very French Raymond Blanc recipe with red wine and tomatoes which is probably amazing. This is pretty damned good though.
Method
Put the wine and sherry in a pan and bring to a simmer for 5 minutes
Put the saffron and paprika in it now.
Leave the saffron and paprika to steep in the wine for about 10 minutes then fry the cumin in olive oil for a minute along with the copped parsley stalks, then add the garlic with a little bit of salt for another 30 seconds.
Pour in the cooked wines, saffron and paprika and bring to gently simmering.
Put the squid in and let it very gently bubble for an hour.
Turn the heat right up and add the cornflour mixed to a paste with a bit of cold water (or col sherry if you are using it) and , simmer for another 5 minutes to thicken and you're done. Stir in the chopped leaves from the parsley and serve.
It's lovely - I cooked it last night to go with other Spanish stuff as a sort of tapas but the rest of it is in the fridge (happy days). We ate it with lovely crusty bread.