I've been writing this blog since 7 months...

Here's another time-related proposition. I often hear native German speakers say "I've been working at ACME since 3 years." The correct proposition in this case is "for", "I've been working at ACME for 3 years.". Use "since" if you're refering to a particular past date. "I've worked at ACME since June of last year." is correct.