Knowing when to buy is critical to maximize returns.Deciding when to purchase a stock is perhaps an investor's most challenging decision. This is especially true with newly public, unique companies in high-growth industries, like Arm Holdings (ARM -0.41%). Arm Holdings stock has more than doubled since its initial public offering (IPO) around nine months ago, and many investors are eager to own this company. But is now a good time?Several factors make timing purchases difficult:We all assume, intellectually ...