I wouldn't be aiming for a set amount of days. That goes for dry fasting more than for water fasting. Just take each day as it comes and watch for signs of dehydration. Just as hunger is the best indication of the need to break a Water Fast thirst is obviously the best indication to have some water. But be vigilant about any other symptoms that may come up. You can get a list on the internet of different indicators- some don't apply so much to fasting but it's always better to be safe than sorry. One they say is good to watch is the elasticity of the skin- the speed it retracts to normal after you pinch it. As the skin dries out this gets slower and slower.