This product comes in 60 square metre packs. You will need enough for two applications 2 weeks apart.
The nematode solution is applied to the branches, the soil under them and the trunk. This should be done once the caterpillars have left the fruit hence the September/October recommended timing.
If there are lots of fruit trees in the surrounding areas (i.e. your neighbour's gardens) the treatment is unlikely to be as effective as the pest will come in from other trees the following year.
When the nematodes arrive keep them in the fridge and use before the use by date. On the day of application get them out and allow them to warm to room temperature. Follow the instructions carefully.