If you are going to decline or accept an assignment, please reply to the sender as soon as you can. You might have to say that you are checking your calendar or something but, they need to hear from you to know whether or not they should be moving forward with you or moving onto someone else.  There is not a rule on how soon this is done.  I would say that the more time that passes the more time that someone will think that you are too busy to make a short reply and let them know that you might be interested in their email.

It is the professional thing to do if you are not interested in an assignment to let them know as soon as possible If you are not interested in the assignment because of pay, date of being scheduled, or any of those details, you might want to ask and see if any of those can be negotiated. Silence can mean that the sender might never contact you again since you never replied to the other emails that were sent to you. You never know when someone can send you an assignment that you would be head over heels for.