SNG Bankroll- Long Term

Hey guys, I've been playing PartyPoker SNG's for a while with good profit. But I've been cashing out too much and busting when I go in a bad streak. I've decided to stop cashing out and start building. How many Buy-ins should I have for 55$ SNG's if I am not going to cash out? (If I get too low, I would move down limits)

I've heard anywhere between 50 buy-ins to upwards of 125.