I've seen a lot of non-native writers using this phrase:

"Since a few days ago I have been doing this..."
"Since some months ago..."

Okay guys, sorry to say, but this is incorrect. Since this is kind of a tricky word to use, here's an excerpt from dictionary.com:
1. from then till now (often prec. by ever): He was elected in 1978 and has been president ever since.
2. between a particular past time and the present; subsequently: She at first refused, but has since consented.
3. ago; before now: long since.
4. continuously from or counting from: It has been warm since noon.
5. between a past time or event and the present: There have been many changes since the war.
6. in the period following the time when: He has written once since he left.
7. continuously from or counting from the time when: He has been busy since he came.
8. because; inasmuch as: Since you're already here, you might as well stay.
So, if you want to say you've been doing something for a few days, say this:

"I've been working on this since Tuesday." OR "I've been working on this for a few days."

If you wanted to say you became something at some point:

"I've been blogging since 2004" or "I've been blogging for 5 years."