Google brought in veterans like Omid early on. However, I believe Facebook waited much longer.
This is a fantastic question, but in order to answer this question effectively we have to define the stage of the company.
Ideation/Validation - Trying to figure out is your product/service solving a problem.
Product-Market Fit - In this context, its when the company has got some early customers but still haven't figured out how to scale the business.
Growth/Scale - You've figured out product-market fit, and you've got a very good handle on if you spend $X you get $Y back.
Now that we've defined that stages, each one of them requires different talent to achieve different outcomes. It also matters what type of a business and industry you are in.
Below, I've made some assumptions around type of business and industry to give response on who to bring in when.
During the ideation/validation phase, I believe someone who has the ability to do customer development surveys would be great. Typically, the founder of the company is expected to do this but if they lack experience or expertise they can reach out to experts on Clarity.fm, and look up blog posts by Cindy Alvarez or Sean Ellis. At this stage, I strongly believe that you should be conservative with the headcount. A tech guy in addition to a the founder is probably appropriate.
In the product/market fit phase, a growth hacker who can test different value propositions, run A/B split tests, Cohort analysis to hone in who wants to buy and why they are buying is great. This is also a great time to start building a strong pipeline of leads. A consultative sales person would also be good to help identify and test different sales processes to see what resonates with your customer.
Scale/Growth is a stage where you've got many of your processes tested and have reduced the risk (as much as you can in an early stage company) of the business. Based on the type of business, you would build your salesforce with an existing pipeline of leads that they can sell too. A marketing team that continues to grow existing customer base while spending small dollars experimenting with new markets.
Again, this information is VERY generic but I hope it provides some value.
It's never too early to bring in experienced people. At an early stage company you're operating with limited resources and you can't afford to waste time making other people's mistakes again.
Bring in experienced people as soon as possible. If you think you can't afford them, try to find people that will share their expertise with you for free.