The ones more elaborated take between 6-10h usually, like the second one above. The simpler ones like the other 3 take between 1-4h I do a quick 10-15min sketch before proceeding to finish it. It usually goes sketch>color>polish, it's pretty straightforward. My sketches look like this usually~

NSFW

oooo thank you for sharing 😊<3 the lines even in the sketch are really confident ! I tried sketching with full opacity brush but its so harrrrddd ＞︿＜ That is really impressive Kan (～￣▽￣)～