To ensure a superior online experience, when your customer selects a time slot, this time slot is "reserved" for 7 minutes for the customer to complete the booking process and confirm their booking. 

If they do not complete the booking in those 7 minutes, the time slot is "released" so someone else can book it. The customer will be notified at this point that the time they selected is no longer reserved for them so they should try again: 

Additionally, 2 minutes in advance of their session timing out i.e. their booking time being "released", they'll be warned that they should complete their booking in 2 minutes to secure their time: 

In our research, we looked at the average time taken for a customer to make a booking via your booking tools. 7 minutes ensures that the customer has sufficient time to make their booking, while ensuring that an in-demand timeslot isn't held for too long. 

You can change the duration for which a timeslot is reserved for your customers in your Booking tools tab, under Customize your booking flow. The setting is labelled Time for which a booking time is reserved.

While doing so, bear in mind the steps on your booking flow. For example, if you require payment, your flow will take longer for the customer to complete.