If your time is flexible, then there is a way (with research of course) to roughly ascertain how much time you would like.
List the top 5 things you want to do. These are ones that you would not want to compromise, whether they be an experience, a tourist attraction or an activity. List them.
Then plot on a map (physically or mentally) - you do not want to spend your holiday criss-crossing, even a city as small as Paris. It will really affect your experience.
Plan 1 major thing per day (2 if they're very close). that is your duration.
Then, around that 1 thing, with your guidebook, map and websites, plan your day around that quartier - other minor activities or attractions, food, nightlife etc.
This is a reasonable approach - you can always reduce from there if you find that in your top 5, there are only 2 or 3 that are absolute MUSTS.
My other point is why go from Paris to Spain. Paris doesn't reflect France, just like every other capital.
the train network in France is pretty good, so go south. Carcassonne, Montpellier, Perpignan - somewhere en-route to Spain, but another aspect to the country.
One of the best things I did between a visit to a friend in Madrid, and another weekend in Paris, was take the train from Madrid to Bordeaux for a few days, and thereafter to Paris.
My favourite area in Languedoc, though, so I would go there. Easy hop over the boarder to Spain.
For non-Europeans, I generally recommend train journeys for European travel - you experience so much more. And it's far more civilised.
