Depends where you are looking as each area of the country is doing or growing something different it all depends on the weather in a nutshell if there is rain everyone is busy if it is dry many pilots will be out doing other types of work (truck driving etc) until it rains. WA spray season is end Aug till about Nov spraying wheat etc. Around this time of yr the girls and boys in NSW, South QLD are usually busy on the various crops.
Its not a job you just walk into it takes a total commitment in the industry to get a start. Jobs are there for those that have the drive and commitment. How long you are a loader all depends on the operator and how well you perform as a loader. Those that put in 150% effort and commitment are usually rewarded quickly, those that slack off and have a bad attitude will be culled out.
Experienced Ag pilots are in short supply world wide and many pilots work overseas in the off season. How in demand you will be will depend on your experience.
If you are curious go have a talk to an Ag operator and have a go at loading they are usually always chasing loader drivers.