To reduce the number of notifications tick the overwrite existing notifications (in the notification action)
I assume by idle time you mean the total time the screen is off...between charges?
To use a timer running all the time the screen is off will significantly drain the battery.
An alternative approach is to use the magic text system time as follows
When the screen is off set a variable to store system time.
When the screen is on set another variable to store system time
Add the difference of these 2 variables, the idle time, to a third variable used to accumulate the idle time.
This third variable is set to zero when the charger is removed and used to generate total idle time in hours & minutes when the charger is started (the system time is in seconds, as is the total idle time.)
This approach won't drain the battery.