I haven't had twins myself, but have 2 close friends who have.
Friend 1's boy/girl twins were delivered naturally at 37+4 at 4lb & 4lb 1oz. No time in SCN or NICU, they were sent home at around 4 days, before their mum who contracted an infection.
Friend 2's identicals were born at 32 weeks & she was flown from Dubbo to Sydney to have them. They were 3lb born & were in NICU in Syd for about a week, then SCN in Dubbo til they would've been 38 weeks gestation.
As far as I know, most prems usually remain in hospital til around their due date, depending on how they are going.

Hope they hold on for alot longer for you