I have a slight variation on this theme running - I share a couple of hobbies with one of my regulars, one in particular needs us both to work together and what has happened is that certain meets have extended & extended over time to be more hobby time than 'fucking' time.
I usually book 2hrs & we stick to that as a specific period of time in the bedroom at her standard rate, the hobby takes place elsewhere in the house & last time stretched to 6 hours of having other fun together for which there is no charge.
As an extension to the hobby we now have a road trip planned which will be all day at an organised event 300miles from home, as I would be booking a hotel anyway she is staying with me & I will be having my usual 2hrs worth in the evening.
I'm sure this will baffle some readers but its working out very well for me as I enjoy her as a 'hobby mate' a great deal