How long do I have to sleep my baby on their back for? At what age is it safe they sleep on their tummy? My 14 month old (is absolutely warm enough) but always rolls onto his tummy during the night, he's done this since he was 10 months old.

Most babies will be rolling & prefer to sleep on their tummy by 4-6months. (some earlier, some later)
Red Nose recommends that you always place baby on their back & let them find their own position of comfort - continue to ensure the environment is safe for your baby.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Rolling to tummy does not necessarily mean baby is not warm enough - it is a normal development stage.