Today a little grammar.  Here are some sentences you might hear if you are talking to French people:

Vous êtes en France depuis quand? (How long have you been in France?)
Vous habitez Paris depuis combien de temps? (How long have you been living in Paris?)
J’habite à Paris depuis quinze ans. (I’ve been living in Paris for fifteen years.)
Depuis combien de temps étudiez-vous le français? (How long have you been studying French?)
Il conduit depuis quatre heures. (He has been driving for four hours.)
Nous nous reposons depuis une heure? (We have been resting for an hour?)
Vous attendez ici depuis une demi-heure? (You have been waiting for a half hour?)

Depuis means “for” when used with the present tense and followed by a time expression.  It is used to show that an action began in the past, but is still happening in the present.  This construction is the equivalent to the present perfect in English; in other words, “has/have been ___ing”.   This structure cannot be directly translated word for word, so be careful!

Now, you may have noticed above that I used both structures “habiter Paris” without a preposition and “habiter à Paris” with the preposition in the above examples and you may think I have made a mistake.  Neither are mistakes and both are not only grammatically correct, but often used and heard by the French and Parisians in particular.  Habiter can be considered transitive or intransitive and therefore can be used with or without a preposition.  Now, where the real controversy lies is in the much-used expression by Parisians “J’habite sur Paris” which is not really considered grammatically correct by all, but has become accepted or not-so-accepted (depending on who you are talking to) common spoken language.  Parisians use “sur” to mean that they live in the “banlieue” near or around Paris, but not in Paris city center itself.  In a way, it’s probably to orient someone who may not know of the small town or suburb the speaker lives in, but will of course know where Paris is located.  Likewise, you can also say “J’habite dans Paris” so the person you’re speaking to knows you mean that you live in Paris city center.