The only reason not to do hiests irl is the cops and threat of permanantly dying. Now, they need some sort of equivalent deterrent in gtao aswell.
Perhaps instead of a time limit, a heat meter.
Eg, the more hiests you do in a specific amount of time, the more likely you will get caught and have to pay retribution, bribes, or loose cars. That could be counteracted by paying for special items which hide your identy, make you faster, ect.