How long between farrier/trimmer visits

About every 8 - 10 weeks. Cally and Oscar both have feet like 'blocks of oak' according to my farrier who said that it's likely neither will ever need shoes as their feet are great. Xavier's feet are pretty good too and he is also unshod.
 
Currently about every 3 weeks, the rate Roxy loses shoes! Argh. It's a fine line for us as Roxy needs to grow enough hoof to stick the shoes on, but every 5-6 weeks. She is shod on all four feet and has dreadful feet despite my endless ministrations (which will surely start paying off soon! SURELY!)
 
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