As you noted, timers will only run for the page they are added to, and only after being started. Once you leave that page the timer stops and will need another behavior containing a “Start/Stop” to begin again.
This being said, what I suggest is the following:
1) Add a helper field to your form. This can be on any page and is intended to be hidden. Set the default value to “0”.
2) Add a timer to each page on your form. Be sure to set the properties so that timer loops.
3) On “Page is Opened”, add a behavior to start the timer. This will be done for each timer on each page (Page 1 – timer 1 start, Page 2 – timer 2 start…)
4) On each timer, place a “Set Field” command in the “Timer makes interval step” behavior. The Set Field will be configured to modify the hidden helper field.
Set the value to “Formula” and configure the formula for “Hidden Helper Field + 1”
5) On the hidden helper field, add to the “Input changes field data” behavior a simple branch where the value is “Hidden Helper Field – is equal to – (your designated number of seconds). On the True path, set your “Show Message”, then add another Set Field that sets the hidden helper back to 0.
This will increment the hidden helper field +1 for each second any one of the timer steps up. Once the hidden helper field reaches your designated time, it will display the message and reset.
If you need further assistance with this, please submit a support ticket.