Start with every 6 weeks and adjust from there. If the farrier/trimmer is taking off a lot of material, then you should probably have them done more often. If it seems like he's barely filing anything off then you may be able to go longer (but not necessarily, as some horses may not grow a lot of hoof but may become imbalanced in that time)
As for chips and cracks, topical treatments will not help with that. Dry is good for hooves, and too much moisture can make them weak. Ask the farrier to put a good bevel or "mustang roll" on the hooves and don't let them get too long. This is the best way to keep them from chipping and cracking.
“The horse is a mirror to your soul. Sometimes you might not like what you see. Sometimes you will.” - Buck Brannaman
"Nothing forced can ever be beautiful." - Xenophon