A pleasant walk. Was up about 3 weeks ago, when I had a conference in Edinburgh. Under 2.5 hours for a stroll from behind Holyrood House, along Salisbury Crags, onto Arthur's Seat then down past St Anthony's Chapel. A lot of the time spent playing with the camera. so, it's easy to fit into a short afternoon.
The only problem is that the summit rocks are incredibly polished; no grip at all in a gathering gale.