No, your faction leader and your Head of State can, if they're good enough, give you a certain number of prestige points each turn, most likely just 1 or 2 points. The buildings and technologies that says "Increases national prestige" each give a certain number of points when completed.
If, for example, your king gives you 1p per turn, that will mean 100 prestige points after 100 turns. Let's say that you during these 100 turns research technology that gives you 250p, as well as construct buildings that gives you 100p, your prestige ranking after those 100 will be 450. The points are thus earned during the game, and they are NOT calculated at the end of the campaign, or an average, or anything of that sort.
You cannot see how much prestige you will get from a technology or a building in-game, you'd have to look inside the game files. In-game it simply says "Increases national prestige" or whatever on their descriptions.
You can check how many prestige points you've earned in the campaign by pressing "M" and looking on the "Prestige" tab.