No simple answer exists other than “before you need it.” In general, the longer you wait to raise any round of investment,

• The higher your valuation is likely to be because you have likely reached more milestones, like generating more revenue, obtaining more partners, producing more product, and so on. Therefore it is good to wait.
• The higher risk you have of running of cash, so unscrupulous investors could take advantage of you by delaying their investment decision until you are desperate, thus forcing you to accept a lower valuation. Therefore it is not good to wait.

Some advisors would tell you to always raise more than you need because you have probably been overly optimistic in many of your predictions. Some advisors would tell you to always raise less than you need (i.e., be as lean as you possibly can be) because your company's valuation will always be higher later and you can raise less expensive money later. Only you can make this decision.

For additional details, see blog post, When Should You Ask for Investments?

Related Question:

How do I change the date of an investment round?