Every 6 weeks for my ponies. Love my farrier, he comes out, trims and lets the horses go in the pasture and I don't have to be there!
I would not allow that with my horses, and won't trim other's horses unless the owner - or representative of - is present. They don't need to be hands-on, but present in case of 'what-ifs'. Especially in this litiginous society, if anything happened to the horse, regardless of what it had to do with me - eg horse goes lame, someone leaves gate open, whatever, and I was the last to see/deal with it without the owner or anyone else present.... it's not worth considering. Another reason for watching the horse move before & after a trim too!