Welcome!!
The time I spend depends on the project I'm doing and what else I have going on. The most has been a couple of hours each night and 4 hours a day on the weekend.
My average would be about 5 hours per week.
As for the time for the 5 x 7 cross stitch - it would depend on a lot of things. How fast you stitch, how intricate the design and how many colour changes there are.
A design with large blocks of colour will be faster to stitch than one with an intricate design.