If you didn’t log the hours, it never happened
When you are good at something (like design) or fast at delivering that skill, it creates the illusion for some clients that your work requires less effort. That myth is the curse of the artist because many people tend to take advantage of creative talent, rather than valuing it.
The best way to command respect for your time is to bill for it. Nothing sobers up a client’s perception of reality than an honest, time-itemized invoice. Discount it (if you choose) so that they can see your generosity. If you don’t show a discount when you choose to give one – it’s as though it never happened. The only thing you will accomplish is establishing a lower price that will be challenging to increase.
Commit to an hourly rate, then stick to it. Use a timer to capture what you put into these projects. Stop the timer when you stop working. Start it again when you return. It sounds simple but if you forget to, you risk the possibility of losing money.