AO - fulltime to parttime during probation

Been agency for 2.5 years, applied for AO comp in late 2024, 2 months ago finally got my placement letter however since applying ive got pregnant and had a baby (just shows you how long it actually takes) ive had to extend my placement until after my maternity leave is over, been told ive to “come back” as full time as thats what the placement is, but been told i can put in for part time straight away, just wondering if anyone has done this and how long does it usually take?