This one has bothered me for a long while.

I like to do estimates thinking on how much time a certain task will take (I'm in TI business), so, let's say, it takes 12 hours to build a program.

Now, let's say I tell Project that my beginning date is today. If I allocate one resource to this task, it means that the task will last 1,5 days, implying that it will end tomorrow.

But right now, that is not what it's doing. I say that the task will take 1 hour, and when I add a resource to it, it allocate the resource at [13%] basis, which means that the duration is still fixed... project is trying to make the task last for a day.

I have, on many occasions, accomplished this. What I do is build a plan based on these rough estimates for effort, then I allocate tasks to resources. Times conflict, so I level resources and then Project magically tells me how long, in days, will it take. But every time I have to start estimating again, I end up having trouble on how to make project work like that.