The simple solution is to make sure you have a good reason to offer for free, gratis. For example, for consulting clients, I have a cutoff of 10-15 min before

The simple solution is to make sure you have a good reason to offer for free, gratis. For example, for consulting clients, I have a cutoff of 10-15 min before the bill starts. That is enough time for me to talk with the customer, scope out a problem, give them quick, simple advice, and prepare a quote. It is a good idea because it keeps lines of communication open and it makes sure I am in the loop when something goes wrong. At the same time 10-15 min is also enough time I can get informed consent for paid work and it's a nice value add. It works well for me, actually. It also allows a little bit of leeway without allowing a lot of sliding.