The truth is there is no right or wrong time.
Here are the phrases you can use if this is your intention.
A trick here is to could use a good goldsmith, so that if the ring is big, they will adjust it.
Many couples choose to exchange promise rings because they are too young for marriage.
Understanding what your partner values make it easy to know what to say when giving a promise ring.
Some couples have the bridal set soldered together before the wedding; In this case the groom can place both rings on her finger during the ceremony, in the proper order.