second chapter of Wheelocks
I've often thought that the more Latin you learn the easier it all gets, so you're going through the most difficult part right now.
As to your question, I didn't use Wheellock, but I imagine you'd want to start reading as much as you can as soon as you can. If you don't understand it, it's no big deal. When you learn some more then you can go back and see exactly how much extra you understand. It'll give you some idea of how well you're coming along. You'll probably need to use the vocabulary at the back of the book or whatever.