Honeywell says that if not in use, best to have it switched off as regards to MTBF. And not Standby, either.
RCA and Bendix, likewise.
Then, there are those that proclaim...it repels birds.
Sorry, not supported with facts
Switch it ON when needed, otherwise, OFF....just as you might with engine anti-icing.
Now, having said this, our weather RADAR sets are a twenty five year old design, nevertheless, they work just like a weather RADAR should, superbly....might be different with new(er) models.
Others might like to comment about these new(er) units.