Does anyone have any idea how often let's say 15-20 red streak come and go?
It is chance, so you can work it out.
The formula for doing so is simply x
y, where x is your probability of losing an individual roll, and y is the number of rolls you makes.
So for example, lets say you go for go for a payout of "2x". Bustabit has a 1% house edge, meaning you have a 49.5% chance to win, and therefore, a 50.5% chance to lose. The probability of losing is therefore 0.505.
0.505
10 is 0.0010787..., or 0.11%. So at a payout of 2x, you have a 0.11% chance of losing 10 times in a row. That becomes 0.004% for 15 losses in a row, and 0.0001% for 20 losses in a row.
At a payout of "3x", your chance of losing is 0.67, so you have a 0.25% chance of losing 15 times in a row. At a payout of "5x", you have a 3.65% chance of losing 15 times in a row.