The short answer: Whenever you want to!
The longer answer: You definitely don’t need 100% comprehension before moving on. 70 - 80% is good enough, or even less if you are losing interest in the lesson. The big thing is not to try to drill every word into your brain. Your brain will learn over time. After a month, if you return to the first lesson you did, you will be surprised at how well you now understand.
Your brain needs a combination of novelty and repetition. The novelty of new lessons keeps you engaged and interested. The repetition is what helps solidify what you read and listen to.
Read and listen to lessons repeatedly. Have at least 5-10 lessons that you cycle through on a regular basis, adding in new ones periodically.
As Steve Kaufmann says, learn to notice. Noticing words, grammar patterns, and sounds is far superior to trying to memorize vocabulary.