By the end of the tenth century, Western, Eastern, and Southern Slavic language families had formed. Russian is part of the East Slavic language family; Belarusian and Ukrainian are the other two languages in the East Slavic family. Two missionaries named Constantine, who later changed to Cyril on his deathbed, and Methodius decided to preach Christianity to the inhabitants of Moravia. However, before they set out for Moravia, Cyril invented the Cyrillic alphabet. The Cyrillic alphabet is based on the characters of the Greek alphabet and about a dozen letters were invented to represent the sounds not in the Greek alphabet.
Under Peter the Great in the early 18th century the letters and forms were simplified and regularized to make the language easier. In the middle of the 18th century there was a need for a written Russian resembling the educated spoken form. M.S. Lomonosov distinguished three styles of Russian. The first style was High Style (Church Slavonic), used mainly in poetics and religion. The second style was Middle Style, used mainly in lyric poetry, prose, and science. The last style was Low Style, used mainly in impersonal correspondence and low comedy. The Middle Style is the basis of the modern Russian language today. Then in 1918 the alphabet was simplified even further to what it is today.
Why Russian? Most people do not think of Russian as an important language, but that couldn't be farther from the truth. Russia is a land of abundant natural resources and where there are natural resources there is money to be made. Plus as the economy opens up for foreign investment there will be investment opportunities for those willing to take a risk. In addition, due to the lack of people learning Russian that means those who do learn it will have better marketability for employment in Russia or for companies that need Russian linguists to translate or interpret.
There are more than 250 million (roughly 142 million native speakers in Russia) people that can speak and understand Russian, which makes Russian the 5th largest language in the world. Russian is the official language of Russia, Kazakhstan, Belarus, Tajikistan, Uzbekistan, Kyrgyzstan, and is also used in other former Soviet Republics. Not to mention that Russian is one of the six official languages of the United Nations.
Learning Russian will also help you understand the culture and people better than some media report on the news. Also think of learning Russian as expanding your social circle. So not only can you talk to your fellow native speakers in your native tongue you can also make friends in Russian speaking territories too. Learning Russian will also allow you to watch Russian movies, listen to Russian music, read Russian books, Russian magazines, etc. Not to mention that Russian will also be useful when visiting Russia.
The possession of another language, is the possession of another soul - Charlemagne
Own only what you can carry with you; know language, know countries, know people. Let your memory be your travel bag.
- Alexander Solzhenitsyn
This is a very individual based question and there are a couple factors that will influence learning time. For instance if you have already learned other foreign languages chances are that you have developed a learning method that is geared towards your needs and desires and will cut down on learning time. If it is your first time learning a foreign language you will probably experiment a little to find the right way to learn. Also if you know any other Slavic language, e.g., Ukrainian, Polish, etc., that will also cut down on the amount of time it takes to learn Russian because Russian is a part of the Slavic family language and while the languages are different they share similarities.
However, if you are completely new to learning Russian and are an English monolingual speaker, then according to FSI (Foreign Service Institute), it will take roughly 1,100 hours to reach an intermediate level in Russian. Of course, this is a rough estimate and may take longer or shorter depending on your aptitude. So if someone was to devote an hour a day to learning Russian in three years (1095 hrs) they would be around the intermediate level and if they devote more daily time to learning Russian this would obviously decrease the time it takes to reach an intermediate level.
This will probably now bring up the question, from a native English speaker, of how difficult is it to learn Russian? Well Russian is definitely harder than learning French or Spanish, however it's not as hard as non Indo-European languages like Arabic, Japanese, Korean, etc. Some people may think the Cyrillic alphabet is very hard, but it's one of the easier aspects of the language. The pronunciation can be difficult, there are no marks to indicate stress, but Russian is mostly phonetic and you'll pick up on and memorize patterns in the language. The grammar can also be difficult at times too. However, people who are serious and motivated to learn Russian will learn to overcome these obstacles and continue on with their progress