@JoeyLiz You can book it within 12 months of finishing your active treatment. Or if you need to have more treatment further down the line, when it finishes, that is the start of the next 12 months. I started arranging mine in Oct last year (my treatment finished at Easter '18) and I visited Thredbo in April this year. It was just terrific - a luxury that I never would have spent on myself otherwise! My brother & sister in law (who looked after me thru my early appointments and surgery) came with us. They'd never been there either.
It is the same with all things - whether a car park or the retreat - first in, best dressed - and they always have a number of dates available to book, not just one! ;)
Give it a go - if it doesn't work out, it's not because you didn't try. xx
Your Breast Care nurse can give you the recommendation - My Onc did for me. xx