Generally it's a good idea to replace oils after first serious days at the track: that oil contents small metal particles from your brand new diff internals (they're also break in) and you're really don't want to keep this oil running.
After that only you can decide how often you will change liquids. I.e. our local champion in both nitro and electro classes changes oils once per season. In case of normal oils and adequate driving they can last long without noticeable degradation. But bad oil can die very fast: i've cooked some noname oil in my ebuggy' rear diff just within one pack!