I think you should have two optional sections for user to configure alerts before timer hits 0.
So they set their time, lets say 2500
then there is an Optional alert section with two areas that can be configured
Sound alert at __x1__ time and then repeat every _x2__ time(s).
Sound alert at __y1__ time and then repeat every _y2__ time(s).
so for an example they could sound alert at 1500 and repeat 0 times
then sound alert at 500 and repeat every minute.
This gives the user the ability to set a "marker" so to speak but then a separate repeating interval.
With your way I could only have one non repeating alert or a repeating one.
So the user couldn't alert when 10 minutes are left and then every minute for the last 3. With the options above they could.
and a user could enter a value for x1 or nothing else or x1, x2. or x1 y1. or x1, y1 y2.