Charging hourly for creative work is a good starting point, but keeps you small in the long run.
A task that took me 3 hours when I first started might take me just 30 minutes now.
Alternatively, you may find it taking more time than you originally quoted the client to find the right solve, leaving you in a place that chooses between doing more work without proper compensation or delivering a final product that might not have reached its full potential.
Pay yourself for all the time it’s taken you to get to this point. You haven’t been working for 8 hours or a couple days. You’ve been working since the day you started.