I've done fundraising for two of my own startups, and advised 4 others. The only answer to this question is "now."

As soon as you think you have product market fit, you should be getting in touch with seed investors. It takes time for these relationships to develop; and in the startup world, time is always running out. Your pitch may fluctuate and adjust while you're out there shaking hands and building presentations. But don't let that uncertainty stop you from meeting people.

You may be tempted to try and perfect your pitch before you start talking to potential investors. But nothing polishes a pitch like repeated failure and feedback. So I'd recommend starting the process as soon as you have a firm enough concept to describe it in a meaningful way.

Also, it's important to distinguish the start of a funding "campaign" and the beginning of a "round." Meeting with investors (the campaign) should be continuous activity until you exit. Once you secure your seed round, you immediately go into series A mode, and so on. The reason being that each funding round has longer and longer lead times. Each relationship gets more and more difficult to obtain.

Actually issuing shares or convertible debt (the round) should , in most cases, begin only when you have lined up more investors that have said yes than you need to complete the round. Put another way, start collecting checks when you have more commitments to than you need to reach your funding goal. That way, you can use your momentum of receiving funds to motivate investors that are on the fence.

The best case scenario is that you oversubscribe your round.

But the moral of the story is starting campaigning for investors yesterday. It always takes longer than you want. So the sooner you start, the sooner you get funded.