how long should I walk my horse to get rid of stiffness?

Each one is different, you can tell when they're feeling looser and more comfortable.

My horse also gets a little stiff when it's below 20 degrees, and moreso if there is freezing rain. We live in the desert and while he does have a shelter, it doesn't do much to block wind. Which is why he wears a blanket too.

I also see my horse about 5 times a week.

I have found that walking in the saddle for a good 30 minutes helps, I don't start with any flexing or lateral work until he's done that. Then I do our workout, which could be 30 minutes to an hour or more, then I cool him out. I find that turning him out in the arena now, after the ride, is more enjoyable for him and he'll move around more and play rather than standing there stiff.

Absorbine rubs after a ride help a lot, especially on the legs.

I have also just switched to a new flex supplement, RapidFlex by Manna Pro. I had him on a Platinum Performance product which was good, but I heard the RapidFlex is better. We're on day 3 of this so I'm hoping it will make a difference.

Many horses that get sore or stock up, you'll see a difference within 20 minutes of work. It gets the blood moving, everything warm and stretched out, and exercise produces endorphins which numb pain.

Easier said than done, is there another place you could put him where he would be out, on his own or with other horses, running around and "being a horse" rather than living in a stall? In my experience, horses in stalls do a lot better if they are seen to every single day and given a few hours of turnout, rather than just being walked. And horses out to pasture do better than those in stalls.

From what I've seen of people who show horses (we do not show at all now), some of them are afraid that dear Sunshiny-poo will mess up his equine beauty by getting mud on one toe, and keep him all wrapped up in stalls and tail bags. Then they wonder why Sunshiny-poo is tending towards weaving, cribbing, stocking up and colic. (Just opinion, folks, just opinion. I'm sure you all are much better than that.)

The lack of exercise from confinement in a stall 24/7 results in varying degrees of muscle atrophy (loss of muscle mass with wasting and weakness), depending on what happens when the horse leaves the stall and on how often that takes place. When skeletal muscles atrophy and lose strength, they easily fatigue and they fail to provide consistent and balanced joint support, resulting in joint stress and stress-induced microtrauma to joint cartilage. This begins the degenerative process that becomes chronic osteoarthritis (OA), or worsens already existing degenerative joint disease.

Walking will get blood moving through veins and arteries to improve blood and lymph circulation and serve as a "warm-up". Accumulated tissue fluid will be moved out of the extremities (stocking-up is resolved) and the increased blood flow through muscles improves their flexibility and ability to lengthen and stretch muscle fibers (improves elasticity).

In order to combat muscle atrophy and prevent OA, exercise must be regular, muscle strength must be improved and mass must be increased.

Simple walking may not be enough to combat muscle atrophy from hours being spent standing in a stall day after day. Adding resistance such as by working in sand or going up and down hills, or the added effort required to maneuver over cavaletti, etc. would increase the muscle strengthening afforded by the exercise.

If OA is already developing, the horse may feel painful and stiff and resist exercise. The more acute resistance exercise you can provide, the better, since it has a direct beneficial effect on cartilage structure. Levels of certain antiinflammatory cytokines in synovial fluid increase after acute sessions of resistance exercise. These cytokines are known to protect cartilage by suppressing the release of other pro-inflammatory cytokines and activating synthesis of new cartilage and synovial membrane cells needed for tissue maintenance. During regular resistance exercise, levels of beneficial cytokines remain elevated, thus reducing inflammation in the joints. When exercise is undertaken more frequently and muscle mass is maintained, joint stability, mobility and flexibility are improved and the benefits protect against degenerative joint disease (OA).

Your horse would benefit most from more frequent exercise. The horse should be gradually conditioned and workouts should stop short of producing muscle fatigue. Continuing to work a horse after muscular fatigue develops significantly ups the risk of injuries.

Depends how still abd stocked up in my option. Try streching his legs from the ground doing some flex work, side stepping from the ground. The little things to help get blood flowing back through his legs do he is comfortable.