As Mike and Ross mentioned, it's not really a good idea.
I have premixed some Testors enamels for brushing, (it's not as thinned
out as for spraying). It shortened the shelf life, but still lasted
over a year. I thinned a few for washes, which is probably thinner than
needed for spraying, and they lasted for months.
One thing to remember, we might be able to thin and clean up our enamel
paints with paint thinner - a.k.a. mineral spirits, but it's not the
solvent used when they produce the product.
Acrylics - I have no idea about them.
In the automotive industry we often had some paint left over after
spraying. We would save it for awhile, but the shelf life wasn't too
long, and we used the proper thinning agent. Should have seen some of
the custom paintjobs on the toolboxes LOL.
I wouldn't premix the paint any farther ahead than a few days or at
most a week if working with enamels. It's not that time consuming or
hard to get the paint ready the day of spraying.