What a bankroll is and is not
Your poker bankroll is money set aside exclusively for poker, separate from living expenses. Playing with money you cannot afford to lose changes the decisions you make at the table, usually for the worse.
Standard buy-in guidelines
Most cash-game players use 20-30 buy-ins as the minimum bankroll for a given stake. At NL10 (€10 maximum buy-in), that means €200-300 before starting. Moving up in stakes is sensible when you have 30 buy-ins for the next level; moving down is sensible when you drop below 15.
Adjusting for rakeback and bonuses
Rakeback changes the effective cost of playing. A player breaking even at the table but receiving 30% rakeback on €100 of rake per month is making €30 per month. Factoring in your expected rakeback when calculating sustainable stakes can allow you to play one level higher than the chips alone would suggest.