long onTime_GB = 10*60*1000;
10 is a value that fits in an int. So is 60. So is 1000. So, integer registers will be used. 600000 is NOT a value that fits in an integer register.
long onTime_GB = 10*60*1000UL;
Now, long registers will be used because one of the operands is a long. All intermediate results will fit in long registers, and the result will fit in a long variable.