The bankroll you need depends on your win rate. You might have a good guess about your win rate or not, but others can only have wild guesses.
A useful formula is
bankroll = comfort * standard deviation^2 / win rate.
Comfort is a term which depends on your personal risk tolerance, and how willing and able you are to move down in stakes if you hit a bad streak. An aggressive player may use a comfort level of 2. A conservative player may use a comfort level of 4. Most people want to use a value somewhere between 2 and 4. Another interpretation of the comfort level is that your risk of ruin if you stay at the same level without withdrawing is about 1/7^comfort, so 2% for an aggressive player and 0.04% for a conservative one.
Standard deviation is a statistical measure of the size of the gamble of playing a tournament. A tournament is about as risky as a fair coin-flip for 1 standard deviation. Your standard deviation mainly depends on the prize structure, so it is about the same for any 9-player 50-30-20 tournament, super turbo, turbo, or regular speed. The standard deviation is about 1.5 buy-ins.
Win rate should be measured in the same units as the standard deviation. You can express both in $/tournament, or % of a buy-in, or buy-ins.
For example, if you choose a comfort level of 3, and you believe your ROI is 5%, then the formula suggests that you use a bankroll of
3 x 1.5^2 / 0.05 buy-ins = 135 buy-ins.
Because win rates are often lower in super turbos than in turbo or regular speed SNGs, you should expect that you will see larger downswings in buy-ins, and you need a larger bankroll for the same level of safety.
If your bankroll falls below this amount, you don't have to move down immediately. However, playing with less than half of the amount suggested by the formula should be viewed as an expense, and you should usually try to move down before then.