Hi Paul, there's a few different part time options available.
There's the pre retirement plan. Starts at 58 and 80%. I suspect you're not looking at that.
There's FW160 aka 80%. There's a seasonal variation, FW160S. Essentially, you work more or less full time during Summer aka high season with reduced flying over winter.
There are other options too, including the 5 months off, 7 months on. Unfortunately, the 5 months off is over Winter and you need to reapply for the time off in the new year. A bit complicated unfortunately.
When can you apply? Usually just after Summer I think, and more importantly, it's business needs so there's no guarantee. Anecdotally, FW160S seems to get approved the most.