I pick my horse's hooves at least once a week. Our pasture is dirt/grass with almost no rocks. I always pick them out before/after a ride. (Or even a training session.) If it was muddy for a while then dries up, I will pick their hooves out then, too. Their hooves are decent, but we have one mare that has really nice hooves. Usually when she is running around the whole cake of dirt will just fall out.
I do pick my horses feet up to look at them nearly daily. If they look bad I'll pick them out.
I pick my horses hoofs out everyday (of course,I'm at my horse everyday)
Since he is outside right now 24/7 on good pasture. He really just cakes in some dried mud and some rocks in his hoof (which is good ,since it is clean dirt and it helps with filling the hoof and creating frog support which is natural and healthy. So it is probably less likely to develop thrush and other nasties than standing in a stall.
I always try to pick them out before I ride (since that is when you can see if you sprung a heel/bent a shoe, pick out the rocks etc .) and since you will be actually asking them to work and carry a rider, instead of putting them back out in the pasture/stall. I think it is more important.
If I can, I pick after too. Then let him back outside to "mosey" and graze. And to let him "repack his hoof for frog support"
If a horse is stalled though. I think you should pick EVERYDAY if you can. Or have someone do it for you.
I don't see my horse every day, so I pick her feet whenever I see her (usually 3-4 times a week). I pick them out before we ride, and I check them afterward. If there's a rock or something in there, I want to get it out of there before it has a chance to do damage. I make it a habit to spend some time just studying her hooves and making notes of any minor changes I see. If I know her hoof inside and out, I'm able to notice problems before they become problems.
I pick out at least every day, no exception. It just seems silly to me to risk a foot just to save 1 minute of riding time. Alli is fronts only and although the fronts pick up more mud, generally its her back feet (unshod) that get the most problems with little stones in the clefts. I kinda thought it was just her being strange but mls got it right for me too!
So, yeah, every day minimum, upto 3 times when riding- morning, before ride, after ride. Although at the moment I need a headtorch in the evenings hehe