Iv had my 250 watt HPS for 2 years almost and I no that the ballast have to wear out some time I was wondering how often should I get a new one?
When it stops working. When they go, they don't work anymore. But I've been using the same ones for 8 years, and never had to replace the balast, just the ignitors.
If you have a continuous grow replace your bulbs once a year. MH bulbs degrade faster than HPS bulbs, but it is a good idea to replace them both at the same time.