How long's a bit of string. You just have to take as much as you can afford and budget for a bit extra on the credit card. Eating out is cheaper than here, especially off the beaten track. I remember buying pizza by the gram in Rome which worked out quite cheap. It can be very hot in Summer you will need to buy a lot of water although I think you can drink water from some of the fountains.
depends how well and how often but I've just come back and found about 10 euros for lunch and 20 for dinner including some wine was plenty. As for souvenirs, how long is a piece of string? Small guidebook for 5 euros, fancy art work for several thousand.