Hi Fobs
Depends on whether the mess is getting to you or not ... sometimes it's easier to let things go a bit, and lower your standards a bit.
My own experience, we employed a cleaner for 8 hours a week when I had my second child ... the hassle of finding someone, explaing how you like things done .... your standard of clean and the cleaners standards may be poles apart made it hardly worth the effort. We subsequently gave up on her after a month.
If you do decide to hire someone, make sure you have enough specific tasks for them to do, especially jobs you don't like doing yourself ... cleaning windows, hoovering, changing beds, ironing etc. Basically things he/she can't get wrong and that you can visibly see have been done.
As regards hours, I would say 8-12 hours a week should be plenty.
Good luck with whatever you decide.