How much should you spend on PPC per month?

Can you explain, in detail, the factors that one should consider when determining how much they should allocate towards Peercoin (PPC) purchases each month? Are there specific guidelines or rules of thumb that experienced investors tend to follow? How does the overall market sentiment and the project's development roadmap influence this decision? Additionally, what strategies do you recommend for balancing risk and reward when it comes to investing in PPC?