I water mine, which are in hanging baskets, twice a day normally, even with the rain we've had the last few days they still get watered at least once a day and we haven't had the temperatures you have in spain. I saw you were having 45 and 46 degrees. Horrendous!

Watering should be whenever they need it, which during summer, will be once a day at least. If they're in a very large pot, once a day might be enough, but in hot sun, twice a day is better.

As for the 'sticky and limp' thing, yes, I'm afraid Petunias do exactly that by about now, which is the reason I stopped using them and switched to Calibrachoa or trailing petunias instead. Might be worth cutting them back, giving them a good feed and keeping them well watered - where you are, they may have time to regrow and flower again before winter sets in.

Feel the compost. Basically if its damp leave it, if its dry water it.

With temperatures like that, I wouldn't expect petunias to do well, anyway. We get temperatures like that routinely here in the desert, and petunias are strictly a winter annuals here.