I've never applied for a Russian visa but mostly rules are close to rules of other countries.

If you will apply for a visa directly to the Russian consulate in London or Stockholm the process will go faster. Actually it is the fastest way.

When I applied for a non-Russian visa in Russia with the help of a tourist agency the duration for such a process was generally of two weeks. So ~15 days is a typical duration.

As for the Russian embassies in other countries the only reliable way to get information is to call to the specific embassy explain your situation in full details and get the actual information directly from them.