Nearly all paints require thinning to some degree (this is a good thing - means you get more bang for your buck out of a quart of paint). How much depends on a lot of things, including the temperature, the humidity, the surface being painted, the pigments (color) in the paint, the type of brush, etc., etc.
You want to be able to brush or roll it on fairly easily (without dragging), maintain a wet edge, have it level itself, and not go on so heavy that you get runs or curtains.
Always use the thinner recommended by the paint manufacturer, even if it is proprietary. If you do not, you risk all kinds of problems down the road.
I highly recommend Pam Wedd's three-part article in Wooden Canoe on painting canoes, which can be found in issues 132, 133 and 135. Pam is among the best at this.
Dan