I am currently a project manager; however I started out as a FORTRAN and Clipper developer, and ran my own business. Small changes are the bane of our existence.
Let me start with a question. How many small 15 minute changes can you perform in a day? Interestingly if you pay attention to your day it will not be 32 per 8 hr day. It will be more like 8! And you, like me, will sit there and wonder where the other 6 hours went. Like me, a typical software developer only thinks in terms of coding time.
Enter the software development life cycle. In reality, there are lead in tasks before development, there are completion tasks after development and there are perpetual tasks that start at the beginning of the project and complete at the end. In all, these other tasks take about three times as long to perform as the actual software development bit. These other tasks are Requirements, Analysis, Design, Testing (U,I,S,A), Management and some QA.
Even as a one man outfit you will perform all of these things in addition to the actual code change. You will most likely have difficulty distinguishing the component parts. It will seem like it is just part of getting the job done. Trust me if you don’t do these things you won’t continue to get much work. Requirements and Acceptance testing are vital customer interfaces but may only be a phone call. Analysis and Design may flow very quickly together with coding as you are intimate with the system and the user experience but, you are still doing it. I bet you never push a change back out without at least viewing the code locally and then again once installed in the customer environment. Full life cycle.
After all this, and in answer to your question, while it seems like the change is quite small you should pay attention to how many of these can be managed in one day. Once you realise the time it really takes you might feel more comfortable billing for an hour minimum and billing more regularly so the customer does not forget about the work. Perception is the only reality, manage it closely.