What may be the time interval between each number?
I'd say there's a 1-2 seconds pause between each number.
How many number would there be in total?
About 7-8 numbers.
Will the difference only be decreasing?
No. For example, the first decrease can be by 7, then it might change to 9. It's up to you to pick up the change in difference. After picking up the change, the number sequence will continue decreasing by an interval of 9 in this case. holdmetight's explained it quite well in his post on Page 4.