Austria: A permanent residence in the country during 10 years is required. Dual citizenship is NOT allowed.
Belgium: requires 5 years of residence, dual citizenship is allowed.
Bulgaria: 5 years of residence, dual citizenship is NOT allowed.
Cyprus: 5 accumulated years of residence in the last 8 years period, dual citizenship allowed.
Czech Republic: 5 years of residence, dual citizenship is allowed since 2014.
Denmark: 9 years of residence, dual citizenship is NOT allowed.
Estonia: 5 years of residence, dual citizenship is NOT allowed.
Finland: 5 years of residence, dual citizenship is allowed. 4 years for refugees, spouses, and people with strong ties to Finland. 2 years for Nordic citizens
France: 5 years of residence, dual citizenship is allowed.
Germany: 8 years of residence, can be reduced to 7 or even 6 with integration and language courses. Dual citizenship is NOT allowed. [Although it's allowed for EU countries and there are other exceptions. This might also be subject to change.]
Greece: 10 years, dual citizenship is allowed.
Hungary: 8 years, dual citizenship is allowed.
Iceland: 7 years, dual citizenship is allowed.
Ireland: Permanent residence in the country during 5 out of 9 years is required. You must be a resident during the year before applying [thanks Vijay Sankaran]. Dual citizenship is allowed.
Italy: 10 years of residence, dual citizenship is allowed.
Latvia: 5 years of residence, dual citizenship is NOT allowed.
Lithuania: 10 years of residence are required. Dual citizenship is NOT allowed.
Luxembourg: 10 years of residence, dual citizenship is NOT allowed.
Malta: 5 years of permanent residence, or 1 year and 1.15 million € [1] (both usually following 5 years of temporary residence as noted by Bence Zakonyi), dual citizenship is allowed.
Netherlands: 5 years of residence, dual citizenship is NOT allowed
Norway: 7 of the last 10 years, dual citizenship is NOT allowed.
Poland: 5 years of residence, dual citizenship is NOT allowed.
Portugal: 6 years of residence, dual citizenship is allowed.
Romania: 5 years of residence, dual citizenship is allowed.
Slovakia: 8 years of residence are required. Dual citizenship is NOT
allowed any more
Slovenia: 10 years of residence, dual citizenship is allowed.
Spain: 10 years of residence are required. This requirement can be reduced to 2 years (but not waived) in case of nationals from a former colony of Spain (it covers a number of Latin American countries and the Philippines). Dual citizenship is allowed for the latter group only
Sweden: 5 years of residence, dual citizenship is allowed.
Switzerland: 12 years of residence (time between age 10 and 20 counts twice), dual citizenship is allowed.
United Kingdom: 5 years of residence, dual citizenship is allowed. Paradoxically, 6 years for EU/EEA citizens - and everyone not free of "immigration time restrictions" 12 months prior to applying