For DS1 it was sometime between 4.5 and 5 yrs old. He was consistently having 2+ hours nap in the afternoon and still sleeping around 7:30pm-7am give or take. He gradually dropped it on his own, some days he missed it out of necessity (ie planned activities etc) and others he just lay in bed awake so we changed 'nap time' to 'rest time' where he could choose to watch tv or do another quiet activity instead of sleep.
His night sleeps remained unchanged.
I will add that we were very fortunate to have had no issues, I realise it's not so simple for most!