
Not as much as you think, I am not a professional artist and therefore most of my work is done as a supplement to income and for the pure love of creating artwork.
Each piece is different of course, ideally I like to work to your budget and as long as the offer is reasonable I'm sure we can do a deal. If you take a rough example of the time it takes to produce a work, say, Picnic in the park was approx. 140 hours, whereas Last Day on Earth was approximately 8 hours. You can then make an offer based on what you think it is worth.
NB: please don't be fooled by any of the time-lapse artwork you see on You Tube, where it appears great pieces of work are "knocked up" in 3 minutes flat :-) it really isn't like that
My more detailed works are quiet time consuming of course and so I tend to work with canvas sizes up to 32" x 24" normally, although of course this could be increased. Most of my drawings are up to A2, again these are not set in stone and I would happily recommend a size I feel would look right for any piece of work.