I am trying to describe a working state in a period of time.
A few years ago, I was running my own business. At that time, I was working 15 hours a day for around 10 months.
... I had been working 18 hour long days over a period of around 10 month.
... I had worked 18 hours a day for about 10 months.
Are those all grammatically correct? Which one is better?
"I had been working" is past perfect continuous, and "I had worked" is past perfect.
Both sentences are used to place the activity or event being described before some other event. Your sentence does not contain any other event, so both sentences sound equally incomplete. To make them sound right, you would need to add a clause or phrase that defines some other event.
For an activity that took place over a long period of time, past perfect continuous is the right choice. Here are two examples that define the other event:
I had been working 18 hour days over for around 10 months before I was able to take a holiday.
I had been working 18 hour days over for around 10 months when I first met Ahmed.
The first one isn't quite right. "18 hour long days" doesn't sound right. The idiomatic thing to say is "18 hour days".
So this sentence should be: "I had been working 18 hour days over a period of around 10 months"
The last one is fine. (For some reason, it sounds slightly odd to me, because "18 hours a day" is extreme, yet "about 10 months" sounds so...casual. I think in practice people would say something like "I had worked 18 hours a day for 10 months straight" or "over 10 months" or something more emphatic.)
A few years ago, I was running my own business. At that time, I was working for 15 hours a day for about 10 months. (This is what you may hear someone say.)
I had been working for 18 hours a day over a period of around 10 months (Grammatically correct and sounds better)
I had worked 18 hours a day for about 10 months. (Grammatically correct but the use of "been" is better)
