The arrival of summer means climbing trips. How would one train to be able to climb as much as possible during a finite period?
—Reagan Chung | Rock And Ice Forum
The only way to improve stamina is to maximize the volume of your training. That means more gym sessions per week than you are used to. Beginners (5.7 to 5.9) should aim to train endurance three times a week, intermediates (5.10 to 5.12d) should train endurance four times a week, and high-level climbers (5.13a and up) may train up to five times a week. You should also consider doing a bouldering or strength-training session each week to prevent losses of power while you are focusing on stamina.
The next step is to consider the split of your endurance training between power-endurance and stamina. Power-endurance refers to sustained climbing sequences, and endurance is anything in excess of roughly 50 moves. If you are training mainly for redpointing or for onsighting short, powerful sport routes, then do more power-endurance and less stamina work. If you are training for onsighting long sport or trad routes, then do more stamina sessions.
To train for stamina at a gym, I would suggest operating three or four letter grades below your usual onsight grade and doing sets of three or four routes in a row without rest. Start with slightly harder routes (three grades below your current best onsight level): Climb the route, lower quickly but safely, then pull the rope, re-tie, and climb again without delay, either on the same route or another nearby of the same grade. Aim for four or five sets of three to four laps, with rest times being roughly equal to climbing times.
Alternatively, pick slightly easier routes (four below your current onsight grade): Climb up, then climb back down an easier route on the same line (two grades easier than the up-climb), then back up again. Shake out wherever possible and climb at a slow, steady pace. Last, you can try climbing on an easy part of the bouldering wall, picking random sequences of movement that get you pumped so that you have to find and use rests to recover. You can go up and down easy problems or simply use whichever holds take your fancy. Clearly, it requires some discipline to keep things challenging, and if you lack this then a superb alternative is to get a partner to point to holds with a stick. Many top-level climbers consider this to be the best way to get fit, as it forces you to do moves that you wouldn't normally choose and to rest at places that feel uncomfortable. Experiment. Try doing four or five stints of eight to 12 minutes with rests roughly equal to climbing time, or you can use a pyramid structure, such as five minutes on, five minutes off, 10 on, 10 off, 15 on, 15 off, 10 on, 10 off, 5 on, 5 off.
Steep, juggy routes will develop a different type of stamina than lower-angled fingery routes, so make sure you replicate your goals or address your weaknesses in your training. Enjoy the burn.