The sentence is:
"Well, I normally do eight hours a day, but at the moment I'm working at least ten hours and some Saturdays."
"Well, I normally do eight hours a day, but at the moment I work at least ten hours and some Saturdays."
What is the best to use?
I think the use of the present continuous fits well in your sentence.
The situation in your second clause contrasts with that in the previous clause. Moreover, the second clause implies that you see the present routine or situation as temporary. In such cases, it is more idiomatic to use the present continuous. So you should say:
....., but at the moment I'am working at least 10 hours (a day) and some Saturdays.
You can also use the present simple if you see this change as permanent.
Well first off, it sounds a little bit weird. It should be:
I'm working at least ten hours a day and some Saturdays."
In the first part of the sentence you're talking about hours and at the end you're talking about days, so you have to add "a day"
Now to your actual question:
"Well, I normally do eight hours a day, but at the moment I'm work at least ten hours a day and some Saturdays."
This is wrong. It sounds awkward, but you would be understood. When you say "I'm" the sentence must end in -ing
"Well, I normally do eight hours a day, but at the moment I'm working at least ten hours a day and some Saturdays."
This sentence works. Alternatively, you could say:
"Well, I normally do eight hours a day, but at the moment I work at least ten hours a day and some Saturdays."
In this context, there isn't really a difference between I'm working and I work.
Your first sentence is:
Well, I normally do eight hours a day, but at the moment I'm working at least ten hours and some Saturdays.
Here, you use the present continuous, but the usage is idiomatic. A native English speaker, particularly a NAmE speaker, would interpret it to mean "I spend at least ten hours at work." It is idiomatic in NAmE to say, for instance, "I'm working at McDonald's," when the meaning is actually "My current job is at McDonald's," and not "As I speak, I am engaged in work at McDonald's."
Your second sentence is:
Well, I normally do eight hours a day, but at the moment I work at least ten hours and some Saturdays.
Here you use the simple present, but the usage here is also idiomatic. As with your first sentence, it would interpreted to mean "I spend at least ten hours at work." Neither is really the best; they both convey the same thought and would be understood identically.
The difference in meaning and usage between simple present and present continuous (or present progressive) often puzzles new students of English. Their meanings, in one sense, are the same: at the current time, some activity is taking place. Although it is not the case in this idiomatic usage, in most cases the present continuous adds a sense that the activity will continue or progress, or that it is habitual.
There is an answer to a different question here which may help you to understand when to use the present continuous.
