The short answer is, “about 3 hours”.
Note that this is oversimplified and even misleading, mainly to prove a point. The actual application of paint is the final step in what can amount to weeks, or even months of work.
It is fairly common for a customer to come into the shop and ask me, “Will you paint my car?” I will usually smile and say, “Sure, next Sunday is good. Bring it in all prepped and I will shoot it.”
“Er, what do you mean by 'prepped', I need to sand it, or what?”
And thence follows a fairly standard quickie lesson on what goes into a paint job. After this, sometimes the customer leaves, thinking that I am either a complete nutter, or that I just don’t want to do it. Sometimes the customer pays attention and ends up getting in line for me to work on his car.
Let me break this into two scenarios. In the first, we have a rust-free car, with no body damage and original factory paint, possibly with an additional re-spray in its history. In the second, we will cover what constitutes the majority of restoration work, damaged, rusted, and poorly painted.