First of all you have to formulate a bankroll management plan that includes the risk exposure of the proposition, variation tolerances, bankroll budget, and perceived collateral expenses. From that information you allocate a percentage of your total bankroll to the daily bankroll to fund your venture for that specific session ---> you quit when you either relinquished your daily budget, or increased your daily budget by the percentages calculated above. Rinse and repeat (for (rinse =0, rinse<1, rinse++))
This is just a pretty well informed way of bankroll management strategy. Just like what iznortboze said in one of his blogs regarding tips on how not to bust in the long run.