That really depends on what exactly you buy and how much you eat. Assuming breakfasts are included in your hostels or hotels, you could get by in most places with late lunch specials—sometimes served until 4—as an early dinner, and supermarkets for picnic lunches and evening snacks, and maybe occasional inexpensive street food—like donor kebabs or pretzel breads. Doing this with your itinerary will probably set you back 15 Euros per day in your most expensive places (Switzerland, London), and maybe 10 Euros per day in your least expensive places.

• Ethnic restaurants are usually cheapest with the lunch specials.
• Look for Lidl, Netto, and Aldi shops for inexpensive beers, cheese, wine, bread, veggies for picnic lunches.
• Ask around at your hostels for tips on good places to eat. For example, the staff at Sir Toby’s in Prague recommended a place down the street that had large portions at very reasonable prices. Other hostellers might also pass on their food and drink finds.

Of course you could cook at your hostels, but unless it’s something very easy, the results might not be worth the hassles and time.