What happens if a rectocele goes untreated for long?

Gets worse: Rectoceles are not "dangerous" but they can certainly be very uncomfortable. They will not get better on their own, but if you avoid straining, they don't have to get worse.

Continued symptoms.: If you are not having symptoms from your rectocele then i would not have it repaired. Noone can tell you exactly what is going to happen with your rectocele. It will not just go away. It may stay just like it is or it may worsen over time. If it gets worse it can still be repaired. Their is no harm in waiting.

Rarely anything bad: There is very little risk with an untreated rectocele. Surgery should be based on your symptoms and how much you are bothered by them. Rectoceles can progress and worsen but this usually takes significant time and the bother issue would likely increase also so most women will know themselves when it's time for repair.