Depends on the shoe and the amount. I mostly wear sneakers and never had any build up. You just put a bit of powder and shake it in the shoe. Humidity will dissolve the powder, some will wick into socks, protecting them as well. If you still see some in the shoe then no need to add more.

Best is to try it and make your own experience. Boric acid is quite cheap.

It dissolves in water. You'd want to spray them down or wash them occasionally, but your sweat would effectively put it in solution and walking would distribute the solution throughout the inside of the shoe. Most probably gets removed by feet and socks each day.