Just going to chime in here. There's a 6 month waiting period between requesting a contract percentage change, and the change kicking in. This allows the company to plan its staffing levels etc. So my guess, 6 months after joining you can part-time.
I'll give some practical examples for schedules. There's a difference between the EUR system and the ICA system.
EUR std = 16/12 in 28 days.
90% = 14/14. There's a slight correction factor here because 10% of 16 days = 1.6, so you get left with 0.4 days. Meaning every 3 months 15/13, because the counter will be at -1.2. The rest have slightly positive correction factors.
80% = 13/15
67% = 11/17
50% = 8/20
There's also a table with minimum time off after working days (not trip) that increases if you do part time. I.e. 100% = 4/2 minimum. 90% 4/3. 80% 4/4. It's possible to work 6 days total however (so work this out in the bid system depending on what you want). Page 145 of the CLA for reference.
ICA is very different. There's a formula that determines free time based on trip length. If a trip is 7 days, you can have 4 days trip and 3 days off. Or 3 days trip and 4 days off. I'll just put some flights here. There's variations, some flights will have a short layover, other a longer one depending on how many flights a week there are (= waiting longer for return a/c to come in).
AMS-LAX. 100% 3/5, 90% 3/6, 80% 3/7, 67% 3/9 OR
AMS-LAX. 100% 5/3, 90% 5/4, 80% 5/5, 67% 5/7
AMS-SIN. 100% 6/5
AMS-GIG. 100% 4/5
AMS-ACC. 100% 3/2
AMS-KIX. 100% 4/6.
AMS-JFK. 100% 4/3
Usually add a day or so for lower percentage.