Bottom-line is that you have to be at a point where the evidence suggests you are ready to spend this capital to create a period of hyper growth connected to a clear path forward to creating a billion dollar business. What that looks like to investors depends on so many different circumstances, namely:
How much have you raised previously and when and from whom?
What were the metrics that you expected to hit with that raise and what's reality since then if the path stayed unchanged?
How many months of "good data" (the data that tells the story that validates the pitch) have you collected and how statistically meaningful are the data signals you're using to sell.
6 months of solid data is really reliable, but three months of good data can create a lot of excitement.
And of course, it's important to ensure that what the team defines as traction aligns with investors' assessment of traction, where there can often be significant misalginment. It's good to be asking this question now directly of investors you think are likely good A investor for you. Adam Nash (CEO of Wealthfront) wrote a great blog post about finding the right investors here: http://13hourstothink.com/2011/11/03/vc-tips-sell-burgers-to-meat-eaters/ …
You also want to make sure that you have filled any gaps in your team required (as defined not by you but by outsider assessment) to really scale your business.
Happy to talk to you in detail about where you're at and try to give you some more specific advice.
Hope this helps set a general sense.