There's scant documentation on the net about it, but generally your gf will have a very difficult time getting by the concierge or hotel security. It's likely she will not be successful because they like to keep a close eye on such things. It's likely your gf would be aware of the situation in the first instance.
If she somehow manages to get past the lobby security, there's still secondary security on each floor supported by cameras (especially in the larger hotels). What this amalgamates to is: they don't like it and they are watching for it. They don't care if she's not 'sleeping over'. They don't care if she's your gf in a serious relationship. It's just the way things work.
If she gets caught sneaking in, she'll have the option of paying off hotel security or paying off the police when they show up. Things can get ugly fairly quickly.
The problem is soliciting and prostitution (and secondarily, the opportunity for staff to supplement their income). You can skirt the issue in one of two ways:
- pay off the concierge each time your gf visits; or
- book the room as a couple
- get her a separate room
I always opted for the latter because it seemed more economical (and more chivalrous). You did not mention the locale, my answer is relevant for Moscow, Peter, Samara, and Stavropol. Probably you can find exceptions, but they would hardly be 'tourist' quality. Hotels in the more remote regions (Perm, Tomsk, Krasnojarsk for example) may have different principles, but I doubt it.
A prostitute working the hotel lobby or bar isn't included in this answer. These girls have on-going arrangements with hotel staff and can go to your room without challenge. Various comments on Trip Advisor like this one, dated 2010: http://www.tripadvisor.co.uk/ShowUserReviews-g298518-d1200684-r77686812-Amaks_Tourist_Hotel-Ufa_Republic_of_Bashkortostan_Volga_District.html,
Trip report from Peter in 2012: http://www.travelpod.com/hotel/Moscow-Hotel-St_Petersburg.html
Trip report from Moscow dated 2014: http://www.tripadvisor.co.uk/Hotel_Review-g298484-d299980-Reviews-Hotel_Cosmos-Moscow_Central_Russia.html#REVIEWS
The 'golden solution' is to get a furnished flat and dispense with hotel complications altogether. They can be surprisingly economical.
Finally, you can consult a forum largely devoted to men visiting their girlfriends in Russia. There are enough current threads such as What are hotels like in the FSU, where people are giving first hand anecdotes of experiences and impressions...
See also Russian Law: 25.04.1997 № 490, Federal Law of the Russian Federation for the legislation from which hotel operators derive the right to set guest policies.