Can't help you with anything detailed, or non-detailed for that matter. I think, though, it very much depends on your artist, what medium he is working in, whether he's painting on wood or canvas or direct onto a wall, where he trained/who he trained with, current fashions in portraiture, whether we're talking a miniature or a full-length affair, and his own habits, bad or otherwise. Things will also vary depending on the sitter and his/her wealth/power/prestige/time available.
For instance, I recall reading that Holbein required only one or two sittings but he made very detailed preliminary drawings, not mere sketches, and he then worked from those. Other artists have insisted on dozens of sittings -- Lucien Freud, I think, was one. Yet others might complete the work from a mannequin dressed in the appropriate clothes, rather than have the sitter him/herself there. In the past some famous -- or then-famous -- artists would only do the face and leave the rest of the body and the background to an apprentice or journeyman in his studio.
I think you could pretty well justify any arrangements, especially if your main character is only the sitter, not the artist -- he/she won't then be expected to know much or even if the artist's methods differ from the norm. Perhaps, though, it might help if you find an artist from the 1600s whose work you like and research him in a little detail.
I've read a couple of historical novels in the last year whose main characters have been artists. I can't actually recommend either one, unfortunately, but you might like to glance at them to see if they help. The first was about Hans Memling, who lived a good bit earlier than your 1600, since he died 1494, but the novel did, necessarily, deal a bit with his art and technique -- The Master of Bruges by Terence Morgan. Also too early for you, the second had Leonardo da Vinci as a protagonist (as well as Niccolo Machiavelli and Cesare Borgia) and though it was mainly concerned with politics and warfare, and hence his mechanical ideas/devices, I think it has him sketch/paint Borgia's mistress at one point -- The Ground is Burning by Samuel Black.