From: Palma de Mallorca
I have made some searches at the Wikipedia (yes, not the most authoritative source, but…)
Berlin 1945: 24 April- 2 May (1 TURN)
Riga 1941: 29 June-1 July (1 TURN)
Kharkov 1943: 7 March-15 March (1 TURN)
Kiev 1941: isolated 16 September, city itself surrendered 19 September, last forces in the pocket 26 September (less than 2 TURNS)
Minsk 1944: Bagration commenced 22 June, Minsk fell 4 July, 8 July surrender of 4th Army (2-3 TURNS)
Rostov 1941: German assault began 17 November-city captured 21 November (1 TURN)
Breslau 1945: 13 February-6 May 1945 (buff, 12 TURNS ??!!!)
Smolensk 1943: “On 25 September, after an assault-crossing of the northern Dnieper and street fighting that lasted all night, Soviet troops completed the liberation of Smolensk” The Smolensk operation had begun 7 September, but the battle for the city itself was shorter.
We have also the battles of Konigsberg and Sevastopol, although it is difficult discern between a “simple” siege and the real fight
In the game, after Leningrad is isolated, you need about 4 Turns to force the surrender. I do not know how many turns would be necessary to force the surrender of a 6th Army at Stalingrad, athough I think it is unlikely it could last more than 5 turns under heavy attack.
The Soviet troops in the Kharkov operation in May 1942 (not a city battle, though) were surrounded during 24 May, and surrendered the “next turn”, 30 May. Well, that would be another (but related) topic, that of the surrounded troops, not necessarily inside a city.
And, as a final note, a pocket can subsist more or less indefinitely if it is not attacked seriously, so it could be a little tricky to deduce conclusions from a real-life pocket without making references to the efforts made to force its surrender (in one game as Russian I still have some surrounded NKVD regiments at the frontier two months after the war began)