Personally, I really don’t wait for a node to be full before starting a new one.
I’d rather slow down the first node for some months than missing out on potential ingress 
No sure what you mean by “income increase slows down for months”. In fact, with a second node, income should increase slightly faster during its vetting phase, in theory.
But we’re talking about so small increases that it does not matter.
More info on how the vetting process works:
Anyway, considering vetting a node can take several months, I tend to start a new node when I feel like the previous node is going to be full in the coming 6 months or so, roughly.
The result is usually something like this:
- First node keeps filling up at full-rate while the second node gets vetted. During that time, the first node is not supposed to be (edit) significantly (/edit) impacted by your second node.
- Second node is fully vetted: At this stage, both nodes are going to share ingress, which means that your first node’s filling is going to slow down substantially, your second node is going to fill up at the same rate as your fist one.
- At some point, you first node is going to be full. At that point, your second node is going to have all the ingress for itself, so it’s going to fill up at full-rate.