The message should stay on permanently.
If the message has popped up briefly and disappeared, then my guess is that it won't be long before it fails.
Get a replacement sensor now and keep it in the glove box ready to replace once it does go.
You don't want to be in a position where you are too far from a dealer to get the part or you have to wait days for it to be delivered - driving too long with the error message will brick the ABS servo. Better to spend the £80-ish (assuming if you go for a OEM part) now or a few thousand £'s later!