Warming up before climbing is crucial in order to have a good training session and to avoid injuries that can keep you from training for weeks.
Raise Your Pulse First
Start with 5 minutes general warm up of light cardio such as jogging in place, jumping jack, etc. This will get the blood pumping to the muscles and stop them from tearing due to being cold.
Mobilise Your Shoulders and Wrists
First mobilise your shoulders and wrists with arm circles, shoulder rolls and wrist rotations are key for climbers. 10 slow arm circles in each direction followed by some gentle flexing and extending of the wrists is all you need to do to get your climbing ready. This type of dynamic stretching is perfect for warming up before exercise and Sport England has some great information on injury prevention in sport that you can read here.
Load Your Fingers Gently
The finger tendons that you use for climbing are an unusual type of tendon and take a slow warming up one. First do some firm opening and closing of the fingers and then do some light dead hang on a big hold (don’t go for a maximum number of reps and don’t go for maximum weight – just simply make sure that your fingers and finger tendons are warm). See also Climbing Walls.
Climb Easy Routes First
After this warm up try to start off with some easy climbs to get into the routes and get into your climbing habits and to get your foot work right and also to get your fore arms ready for the pump you will get climbing.
Cool Down Afterwards
To prevent injury when coming down from the wall, complete a 2-3 minute cool down consisting of gentle stretches of your forearms and shoulders. Hold each stretch for 20-30 seconds and release. This will help remove lactic acid from the muscles and keep you loose for your next session.
So in short a warm up can take around 15 minutes to do before your session to get the most out of it.