Teafortwo, bless you for thinking as you do - the question is indeed how long will your pup MIND being left for rather than how long you CAN leave her for.
Pups sleep a lot, as you know, but quickly grow out of that habit and Cockers are bright dogs with a working background, as you doubtless also know, so will become easily bored, which is when you get the problems of chewing and whining/barking if you are not careful.
If its at all possible I'd advise that you take some time off of work (if you are a working Mum of course) to be with her in the first week or two, both to settle her in and to get to know her pattern of behaviour. That should give you a far clearer idea of how long you can leave her than I ever could.
To an extent, although you will need her to fit in with your family arrangements, let her guide you. You will soon learn how much time she needs for playing and exercise, how much quiet time she needs and so on. As she grows be prepared for plenty of long walks, especially as you live in an apartment, and provide her with lots of safe toys - Kongs are indeed brilliant.
I found it interesting and surprising that in France there is little take-up of dog insurance policies. I have no idea of the cost of vet treatment over there but here in the UK I would certainly recommend insurance. Are your vets fees lower than ours to the extent that you don't need insurance, do you think?
Oh... and... as much as I am an advocate for adopting rescue dogs rather than going to a breeder, I am SO jealous of you! Cockers are adorable. I hope that you will have many long and happy years with her.