Returning something bought online

How many days do I have to change my mind?

What the law says

For a distance or off-premises purchase, you have fourteen calendar days to withdraw, without giving a reason (Art. L. 222-9).
For goods, the period starts the day you (or someone you chose, other than the carrier) receive them; for an order delivered in parts, from the last item (Art. L. 222-9).
For services, it starts the day the contract is made (Art. L. 222-9).
If the seller did not tell you about this right, the period is extended by twelve months (Art. L. 222-9).

Tell the seller before the period ends, with the model form or any clear statement (Art. L. 222-9).
Some exceptions apply (for example made-to-measure or perishable goods): check before sending back.

By practice area, language and town, in a neutral order.