Changing heads
If you have to ask if head needs replacing—it usually does. Make sure tuning rods have washers.
There is no rule I've found to determine when to change heads—after years of playing I can often
tell when a head is going, and when I have to get ready to change a head. It depends on how
hard, often and long they are played. If you play them consistently like for a few hours every week,
I'd say change them every month or so. Unless you play lightly & have perfect technique. I once
heard someone say retuning may make a head age quicker—but you have to retune whenever
playing has stretched it a little. Or if you play in a different room sometimes and the drums sound
different. Or when it detunes from playing. You can't think 'I won't retune my drums because it
may affect head life'—sound is more important—retune when they need
tuning. Having the head too loose will reduce it's life more.

When in doubt—change the heads, if the drums sound the same with the
new heads as they did with the old ones, save the old head for emergency
use. Usually you'll notice a big improvement—which will let you know to
chuck the old head. I have seen jazz players who hit very lightly, and also
aren't bashing away on tons of fills, so I can see a head lasting a while.
Plus—jazz players tend to hit properly, putting less stress on the head.
Just as proper technique reduces stick breakage, it lengthens head life.
So change heads when needed ! ! ! And learn to hold the sticks properly.