I think it depends on what you intend to use your Patreon for.
You can use it one of the following ways:
• As a blog where people can tip you. So you can just post things for everyone to see and readers can support or not.
• Early Access where patrons see the posts first before the public. Sort of like a "Wait-for-free" system. Readers can wait for it to be available or pay to unlock it faster.
• Fund projects, like asking for a set amount a month or reach a goal to get a new tablet. I've seen creators use this system to pay for the next comic page.
• Reward Tier system. This is where you need to be careful because you don't want to take on more work than you can handle.
If you want to use it as a blog, you can start whenever you want! The more content you have, the more people can look at and decide if they want to support you (which would be a bonus for you).
Using early access, I'd start as soon as you know what you want to make early access. Or you can start using this at any time, even if you start posting as a blog you can introduce this later on.
If you indeed to use it for backing up projects or funding, I'd wait till you have more of a following. That way you don't have it sitting there and being another thing to keep track of when there's little to no activity. You can also wait till readers start to request it! Then you'll know that someone will join!
As for what your friend said, I think they meant it in a way of spreading yourself thin. If you do posters or personalized icons, be sure you have time to make those! I know someone who spent a good week every month just trying to get in contact with patrons and talk about what icons they wanted before even drawing them. If you do posters yourself, you'll have to make time for printing, shipping and more time if something happened to the package.
(I spread myself way too thin at first, which took time away from actually producing comics)