Amount of spray and size of bottle has an effect on how long you are in the nitrous. The later you wait in the run to start spraying, the less mph gain you will see. However, the longer you spray the less bottle pressure (less nitrous) you have at the fast end, slowing you down. It's a balance. In addition, EGTs always rise during the entire run on nitrous. Time wise, the longer your spray and bigger the shot, the greater chances you will heat the exhaust valves up enough to detonate. Once again, it's a balance.
I have on hand multiple bottles when I race. The number of runs per bottle is based on your shot size, bottle size, and how long your spray. And once you drop below 1/2 full, it is near impossible to keep any reasonable amount of pressure on the fast end.
What size shot? And bottle?