Most common are 50% and 75% with either a block of 2 weeks or 1 week off every month on a fixed pattern. Able to apply for it as soon as you start but no guarantee you get it quickly. During covid PT was granted rapidly, not sure what the current climate is like.
Different part time patterns that are effectively 1 month off, one month on or 1 off every third month are also available. They can only be bid for once a month and new contracts in this pattern haven’t been granted for a while.
You have to return to full time working after a new type rating for consolidation which is also something to consider.