I am using U-POL UP0745. The first time i mixed it with what i thought was the right abouny of hardner it got hard after about a min of time. I have tried reducing the amount of hoarder but it still seems to get unworkable pretty fast.

What is a normal about of time for filler to get to the point of being unworkable?

It all depends on the amount of hardener and how hot it is.
It takes some practice to get it to where you want it.
Ideally, I like it to set up just a few minutes after spreading it.
Sometimes I cheat and use cool filler (stored in the air conditioning)
to extend the time but still set quickly once it starts.
I even apply a heat lamp sometimes. (I'm very impatient)