How often should I repaint just to keep it looking fresh? Should I wait until the old paint start to peel and crack or should I repaint it every 2 to 3 years regardless if the old paint is still "quite fresh" or not?
It should normally be many years, if not decades, before paint starts to peel. Cracking may occur as the house settles, but that is usually due to the structure shifting and not a poor paint job. If the paint was not applied properly (eg, surface not prepared correctly), it may peel earlier, or this may indicate another major problem such as moisture in the walls.
So to answer your question: you should likely re-paint when the house is looking dated, the walls are scuffed up or damaged beyond cleaning, or when you're sick of the color. Between 5 and 15 years is a reasonable expectation of time between repainting, in my experience.
To have the longest-lasting paint job, the keys are good surface prep and good quality paint (basically just not the cheapest stuff). There's also another question with some good tips about painting.
Though we don't give decorating advice here, if you are going for a long-lasting paint job, keep in mind that there are colors that seem to always be current (I guess the "neutral" colors) and others that while they may be trendy today, a decade from now someone will look at them and say "wow that is so 2010's" just like today you might do that for the 90's or 80's.
Many people paint neutral colors before selling as it does look new, and some potential buyers can be negatively affected by strong colors they don't like or that make a room look dark or small.