Personally the way i handle it is when i start working with a customer the agreement is that i will support what they pay for. On occasion I will 'give' them a free snippet. In addtion to the size of the project i also take into account the purpose. If i am writing a very small peice of code that will end up being critical to what they are doing I charge for so that it falls under the supported rules. If it something that will make someone's life a little easier that doesnt require too much time I give it to them in the same spirit that some hardware vnedors send me t-shirts and stuff in. Hoping to build some loyalty so far it has worked out well.
Comment on Re: The real cost of freelance writing.
I also prefer to make it clear that I have the option of no longer supporting it if I give them their money back and to make it very clear that a change in the requirements/specification means a new project with renegotiation of fees and support arrangements.