When a registrant starts the form process, they are actively monitored in the system's memory. This tracks and reserves any inventory they select while they are incomplete and prevents overbooking. If they stop going through the form and that time limit is reached, the items they selected (agenda items/sessions) are canceled and returned to the inventory pool. As they take up system memory, there is a limit on the number of spots available for each form. This can range from 150 - 900 slots based on the current system resources. Once this limit is reached, registrants will see a notice on the form that it is unavailable, and if they wait, it will check every 10 seconds to see if a spot has freed up. When the Plan > Options > "Incomplete registrations count..." setting has a value of 5,000 minutes, if an incomplete reg does not continue through the form to the Confirmation page, it will take up one of these system memory spots for 83 hours.
The system default of 10 minutes is usually enough; if more is needed, increasing it to 30 or 60 minutes is often all that would be necessary. When setting the time, you need to weigh how long you think a reg may pause in the form, how long you want that inventory to remain unavailable to others, and how many people you expect to register in that same timeframe. The higher the time you set, the longer the system allows an incomplete reg to pause on the form but limits the number of regs that can access the form simultaneously. The higher this is set and the more people you expect to register, the more likely you will run into instances when registrants are told that the form is currently unavailable and that they need to wait to register.
NOTE - Proper management of the above settings for the event should help avoid this ever becoming an issue.