A ballast should last a long time. However, sometimes a bad bulb or other outside factors can fry it overnight. You just never know.
However, you have another issue. An electronic MH ballast is a lot more efficient than a magnetic one. I measured the difference on ones I had many years back. The magnetic ballast was about 65% efficient. The electronic was about 95% efficient.
In doing the calculation with the electricity rates at the time, I found that the more expensive electronic ballast would pay for itself in about a year. As I recall, the electronic ballast costs about twice as much as a magnetic ballast.
I would say that if you plan on using a magnetic ballast more than a year, your best choice is to replace it with an electronic ballast.
Of course, if money is no object, you can put that cost toward that new super delux LED lighting you've been wanting.
