Based on my own experience only, so nothing scientific. I have read so far some stories by Чехов without too much difficulties and I am currently reading Империя должна умереть (not a novel, but the level should be similar)

In grammar, you need all the basic grammar but you also need to know about the various participles (active present / past ; passive present past ; gerund present / past). You don't need to be able to form them or even use them, but you need to be able to understand their meaning.

Concerning the vocabulary, it's a bit more tricky because I expect (ands so should you probably) to have to use a dictionary for a couple of years (after 18 months of studies, my passive vocabulary is only around 10k words and it's obviously not enough to be comfortable).

It's weird, when learning Italian it tooks me about 6 months to read Umberto Eco, about the same time to read Carlos Ruiz Zafón in Spanish (two easy languages to read for a French), it took me a year to start reading Thomas Mann in German, about the same time in English, but I expect two or three years to be comfortable in Russian (not the classics, but I want to continue with Чехов (I simply adore his stories) and I feel like reading Борис Акунини (Фандорин and his История Российского государства). The classics are way back in the future, if I live long enough.