Generally you repot a phal when it is growing new roots and leaves. If the spag. starts smelling bad you can repot. I cannot raise orchids in spag. and plastic pots or I rot all the roots off. Of course, I live in humid Florida. I know a lady here that repots all of her phals every spring. I repot mine when they begin climbing out of the pot. I use a standard mix with packing peanuts at the bottom.