There are a few items on a speargun which are basically consumables.
What I mean by this is that they wear out relatively quickly and will need to be replaced several times over the life of the gun.
The first thing that breaks regularly is the mono line that attaches the spear to the gun.
You can buy a crimping set along with a huge reel of mono line for hardly anything, and this is by far the best way to go.
The next thing that breaks regularly is the rubbers on your gun.
This includes both the firing rubber (s) and the rubber at the end of your mono.
Most people refer to this as the bungee rubber.
This is a very important piece of rubber, as it allows the fish to tug the spear without putting direct stress on the gun and the mono line.
This rubber provides a bit of 'give' for larger fish, but if it breaks then you have nothing holding the spear to the gun anymore.
What this means is that if the rubber wears out and you shoot a big fish you have a chance of losing your spear! Make sure that you check this regularly.
Tug the rubber as hard as you can to test how perished it is, and then have a look for cracks and poor quality rubber.
The other two rubber (s) are less important (although if you break them then you can't fire the gun!).
If you have two rubbers then you have a good chance of still being able to shoot fish, because you just use the other rubber.
I normally just wait for these to break (they make a bang underwater but it's not an issue) but some people replace them when they get perished.
Depending on the quality of your rubber some may last for a long time even when they look very worn out.
Just check the rubbers regularly, and replace them if you are going on a big trip (assuming they are worn!).