I always provide a quote that sets out both cost and delivery date/timeline. To prepare the quote, I need to review the document in question, even for regular clients, as every project is different. Once the quote has been agreed and the finalised document has been sent to me, I can start work.
Usually applies to straight textual translation.
Usually applies to proofing and revising and also to translation when extra formatting and/or checking is required.
A few clients prefer this approach. Calculating a flat fee means it is particularly important for me to check exactly what the client requires.
I look forward to hearing about your translation project!