Well... Im REAL bad with oilchanges on my snowblower!! I think I went 5 years before without changing it.
That being said... I would change it once at the end of the season. that way you get all the water, and acids out of the crankcase before it goes into its long summer slumber. For oil I ALWAYS use synth oil in my snowblowers. It cost more, ya... but realy you only need a qt or two once a year..It makes starting in the cold a LOT nicer!! Also holds up well to the heavy load a snowblower lives with.