Across the training tracks, excluding No Gear, we are always building or maintaining strength. The approach to lifting we use is a "progression system"- by which we mean a system that lets you know when you, as an individual lifter/athlete, are ready to add weight to the bar.
The rule of thumb is the "5-10 Rule." Meaning If you can't lift it 5 reps, it's too heavy. And if you can do 10 or more, you could be lifting heavier. That right there is the progression rule. Simple. Like our motto, simplicity and hard work. That's what will take you places.
Anyway. The weight you're working at is, aptly titled, your "working weight." Once you can do 10 or more reps at your working weight, it's time to add weight. We recommend no more than 10 lbs to a lower body lift, and no more than 5 lbs to an upper body lift.
What about when I fail?
If you fail to get 5 reps for a weight, or you are stuck at the same reps for 3 sessions in a row (for example, squat gets stuck at 305 for 3 squat days in a row). Time to deload. To deload, take about 10% off your working weight. So the squat example- if you got stuck at 305 and it came time to deload, you'd be knocking 10 percent off (we'll round to 30) and go down to 275. Once you're there, your all out set should easily go past 10 reps. And you should feel free to go past. Go to 12, 15, whatever you can get. So long as your form is decent.
Which brings up my next point. Form. This is most important on squat and deadlift. For deadlift, you should either film yourself or, ideally, have a partner watch you. When your form goes south, time to stop. If you can't lift the weight with good form at all anymore, time to deload. When you drop weight, you can get your form crisp and clean again.
So in summary, build reps, then add weight, repeat. When you get stuck, drop weight, blow the reps out of the water, and start progressing again.
Where you will see this change is to expand and contract with your conditioning. But that's another story.