[Mock8800]

From what I could tell It's based on how long your members are on from the point it starts to the point it ends. Thus if you have started Rob a small bank you need some x number of minutes for it to succeed. So if you have a 10 man gang and everyone gets on every now and then you'll fail. If you have a 9 man gang and everyone is always on then you'll succeed. So if you have more members it's easier for you to get x number of minutes, but it's all speculation.