For both your sake and mine, I prefer to charge per project/contract instead of per hour/day.
What a client is paying me for is my effort and "brain space" in designing and the end product itself, be it an online store or a business card, NOT how much time I spent on the project.
If I finish the project earlier than expected, that should be a bonus for the client since he gets what he wants done quicker. It's not a bargaining chip to lower the contract price because I spent less time on it.
Likewise if I finish the project later than expected, it's not a chance for me to ask for more money because I spent more time on it. If I take longer than the agreed time frame, that's my problem.
Just let your word 'Yes' mean yes, and your 'No,' no. —Matthew 5:37
If the client insists that I charge per hour/day, my rates are $50/hour, capped at $250/day.
However, I don't recommend this since I have wifely and motherly duties to care for as well. The client has no means of monitoring whether the hours/days they are paying me for are solely devoted to the project or not. This is why my daily rate is capped at $250/day since I can only realistically work for max 5 hours within a 24-hour period.
For ongoing projects though, I recommend a monthly support subscription fee instead. (See below)
For ongoing projects, such as web hosting, ongoing updates, ongoing technical support, online store maintenance, etc., my rates range from $30-$150 per month depending on the work required.