You can say "I am working here for a month" though it doesn't mean "I have been working here for a month" Why are you here? I thought you left last year. I'm working here for a month. (I've been assigned to work here for a one-month period. Maybe I've only been here a day, maybe I've been here three weeks, but the period I am working is one-month, and I'm currently somewhere in that period.) I came back. I've been working here for a month. (I started working a month ago. There's no indication how long it will last.) To say "I'm working here" when you mean "I've been working here" sounds bad, though if you have an accent and there is some context, people might understand.

Mehrdad, when you say "I am working here" you are using the present continuous tense, which means that the activity is happening right now at this very moment. The correct way to express what you want to say would be to use the present perfect continuous tense "I have been working here." This, then, indicates that you started working at the place in the past and the working has continued to the present for one month or two years and is still happening and will still continue. If you don't want to indicate that the working will continue into the future you could use present perfect tense and say, "I have worked here for one month or two years. This would indicate that the working started in the past and continued to the present but is completed at the time you are telling someone about how long you've been working. It's really a matter of emphasis. In either case you are still working at the place. The difference is whether you want to emphasize the fact that you have been there for a month or two years or that you have been there and will continue to be there.

My take on this is that it is not absolutely clear what you trying to say. You can say " I am working here for either a month or 2 years" although I would prefer " I will be working here for either a month or two years. (you could leave out "either" although I would include it). Perhaps you are undergoing a trial period of a month and if it successful you will stay for 2 years. You can't say "I have been working here for a month or 2 years" as they are not possible alternatives as you must know how long you have already worked. You could use it about someone else when you don't know the answer.

The question is about using time periods with the present continuous form.

I am working here for a month or I am working here for 2 years