Drop-ins being longer than 30 min?

I have a new client who would like something longer than 30 min (1 hour - 3 hours depending on day). Would it make sense to have them book a drop-in visit with me & I stop the timer at 30 min? Or keep it going for as long as I'm with the dog? And then just add an adjusted cost during the booking for the extra time? If anyone has had similar situations can you let me know how you went about it? Thanks!