Replacing HPS Bulb?

I have a 600 watt digital ballast. How many grows before I replace it.

Is Hortalux that much better than Plantax?

Do I need a digital bulb with a digital ballast?

Thanks to all & Peace

PIF

They generally last quite a while in a digital ballast. I use a mechanical ballast 1000w and my current bulb has been used for almost 3 years. Bought a light meter to test how its holding up and it still pumps out the same lumen's as when it was new.
Haven't seen much difference between expensive bulbs and cheaper bulbs.
Digital ballasts use the same bulbs as mechanical ballasts.