I normally require payment in full, before starting your project. In some cases (for larger projects) I will allow the project to be split into more than one payment - this is something that would need to be negotiated before you place your order. If I were to take your money and run away, it's very easy for you to get your money back, by filing a claim with PayPal. However, it's very hard for me to enforce payment, if a client decides to walk away or not pay at the end. For those reasons, I normally require full payment at the start.