I am unable to load the web pages of the Russian Ministry of Foreign Affairs, which is the most authoritative source for your answer, but as a second-best source I went to the travel information site of the US Department of State. On their information page for Russia, they say:
Under a bilateral agreement signed in 2012, qualified U.S. applicants for humanitarian, private, tourist, and business visas should request and receive multiple-entry visas with a validity of three years. Visas issued under the agreement permits stays in the territory of the Russian Federation for up to six consecutive months. (Please note that other types of visas are not part of the agreement and those visa holders should pay close attention to the terms of their visas.) You must exit Russia before your visa expires. The maximum period of stay is shown on the visa.
(emphasis in original)
So, if your visa is a three-year visa issued under this agreement, you can conclude that you are allowed to stay no more than six months on each visit. That doesn't necessarily mean that you can re-enter Russia for a subsequent six-month visit immediately after ending a previous one. Instead, if you want to stay in Russia for longer than six months, you should investigate a proper long-term visa. The place to ask about that is Expatriates.