How long does it take the vagina to tighten after sex?

When arousal ends: When sexual arousal ends, then the vagina will return to its' previous size.

The next day: During normal excitation, the vagina lengthens, lubricates, and creates a barrel shape to receive penile penetration. The act of sex further stretches out vaginal tissue, and all of this is normal. The vagina is very resiliant tissue (as evidenced by its ability to return to a smaller caliber after delivering a baby). Many women like to wait several days between sexual encounters to get tone back.

Very fast: The circular muscle around the vagina contracts very quickly back to previous vaginal capacity after intercourse if there are no vaginal tears.