All of the 12 Great Feasts (and the Dormition is one) are specified as Class I or Class II feasts. Feasts of the Lord are Class I, a All-Night Vigil is prescribed. Feasts of the Theotokos are Class II and also require an All-Night Vigil (some well-known saints, like St. Nicholas, are also a Class II feast). Some Class II feasts offer an option for Great Vespers and Matins separately or to serve a Vigil, others require a Vigil. Theotokos Great Feasts are of the latter.
My home parish always serves a Vigil for each of the 12 Great Feasts. I once attended the Vigil for Dormition at Holy Cross Monastery (ROCOR) in WVa. The service was about 4 hours. At my home parish, Vigils generally take between 1.5 to 2 hours.
Also, Arachne, our chapel served the Vigil for Dormition this year, and it also took us about an hour and a half!