I think my son was about 6 when i used to pop in and out to check, i don't think i left him completely until he was 9 or 10. i think it entirely depends on the common sense and general awareness of each individual child. My son is now 12, he would be mortified if i went anywhere near him at bathtime!!!
I have two younger daughters (9 & 5) who shared a bath up until earlier in the year. I now let the older one bathe by herself, but stay in the room with the youngest.