For how long should someone stay to visit Paris?

Kitkat asked: I have a friend in New York, who want to come to Paris for the first time in her life and plan to stay there for only 3 days. What do you think of this duration?

From the Philippines, how much would it cost to take a backpacking trip across Europe?

daeneluna asked: Also, what is a good duration for a backpacking trip in Europe, provided you wanna see a lot of countries and a lot of places? Would a month be enough?

Wish to go on a Europe trip this summer.Can anyone guide with good websites for cheap tickets?

Aishu asked: Destinations to be covered: Italy, Swiss, Venice ( UK optional)
Period: May-June
Duration: 2 weeks

Me and my husband stay in california and wish to travel cheap.
Can anyone suggest good websites for information on Europe trip or to buy tickets from US to the destination?

Thanks!