So if we go by my OP example. Someone you've known for over 2 years and I'd say get along well, isn't necessarily asking for a discount but asking for a shorter meet (quickie 15kins etc) would you consider it?
I live where I work and it's a residential street, I get on with and chat to my neighbours, I have the business model I have because it works for me.
So 30 mins is my minimum booking, I wouldn't change that and indeed I haven't ever changed it for new or regular clients, however as I've told a few clients, 30 minutes is my minimum bookable time, if you choose to leave early that is entirely up to you but that is the lowest fee I will work for, I still have to shower, dress and get ready just the same for any length of booking.
If it was someone I see regularly while I was on tour I might consider it.