There isn't a timeline.
I've met developers and designers that were making 5 figures/mo. within a couple of months of learning. Similarly, I've met people who've been developers and designers for literal decades, but would never be able to land a 4-5 figure client on their own. The determining factors are typically "how much value are you actually providing," and "can you sell yourself, your agency, and your products/services well enough?"
I'm a full stack web developer that recently sold a digital marketing agency. That said, to give you a general idea:
* At 3 months of developer experience, I was taking freelance jobs in the 300$-800$ US range (HTML, CSS, basic JS; basic website redesigns)
* At 6 months of developer experience, I refused any job under 1500$ US
* At 1 year of developer experience, I was researching starting an agency, and typically refused any job under 2500$ US
* At 5 years of developer experience, I started an agency, landed quite a few clients, and then sold it 8 months later for 12X,XXX. I'm now working on my next app/business.
Cheers.