I’m bilingual. My two mother tongues are English and Bulgarian (which is a Slavic language, similar to Russian).
I started learning German 7 years ago, Russian - 4 years so. Although I moved to Germany last year and have a C1 certificate, it took me a huge effort to master it and finally become fluent.
It took me about 2 years to do that in Russian. I must say, Russian grammar is more complicated because it takes some effort to practice the cases and actually learn them and be able to use them!
In contrast, German has quite unpleasant “der, die, das” noun genders, which can slow down your progress drastically. The passive form (“Passiv”) is also not easy to understand and apply.
Finally, I would say that German is harder than Russian BUT it depends ONLY on yourself how fast you will master a language. For example, my friend managed to start from a complete beginner and get a C2 certificate in German in 13 months.
