Time under tension is the total duration a muscle is kept under load during a working set, determined by how many reps you perform and how fast you perform them. In High-Intensity Training it is a consequence of controlled reps taken to failure, not a number to chase in itself.
A controlled cadence naturally raises time under tension, because you are not throwing the weight. But the target is momentary muscular failure, reached with honest form — not a stopwatch reading. Chase the endpoint and the tension takes care of itself.
Failure Point is a training logbook, not medical advice. Consult a physician before starting any intense training program.