I haven't been to all those places, but here's my humble input on a few:
Of course, at the end, it really depends on what you're interested in doing (living, hanging out, sightseeing, arts, party, bars/pubs, restaurants, etc.). That will really drive how much time you spend in each city. Just remain flexible, as you start your travel, you'll have a better idea of how you prefer to spend your time travelling, and you can modify your initial plans accordingly:
london 2weeks - you don't really need 2 whole weeks. if london really interests you, then I think 5days to a week would suffice.
Paris 3days - you probably need at least 4 days in Paris. if you like the arts, then you might need more time, at least a week. you could spend a whole week just at the louvre!
Prague 3days - i would say at least 4 days
Vienna 3days - 2-3 days, unless you're really into music, then you could immerse yourself in the symphonies, etc.
Venice 2days - 2-3 days, especially if you're there w/ a sweet heart... i just think that despite the touristyness, Venice is sooo beautiful! plus, you can do the little islands off of Venice too.
Rome 3days - 4-5 days at least, there's so much to see and do!
Florence 2days - 2-3 days at least, especially if you like the arts, so many museums!
Barcelona 3days - 4-5 days, especially if you like to party!
Madrid 3days - Madrid has a few wonderful museums, parks and cool little places to hang out in Huertas, that could take you 2-3 days. but just like Milan, it's a place where you need an insider's tips on where to go and what to do, then you could spend more time...
hope this helps!