Most commonly grows a bit, then stops.
Rarely it can continue to grow until big patches are pigment-free.
Physically harmless but can be mentally traumatic and lead to the sufferer dreading summer as 1) they’re more prone to burning and 2) the pigmented skin tans so the vitiligo becomes increasingly obvious and they have self-confidence problems.
No effective treatment but light therapy may help. I’m not a sufferer personally but, in the experience of the sufferer I know (sample of 1 in West Yorks), the NHS doesn’t deal well with it.
Edit: referral to a specialist dermatologist can help to show the extent of it – uv light can show where pigment is being destroyed but isn’t yet visible in daylight.