How important is it and how often should you paint the house interior?

For the most part it is not essential at all, it is mostly decorative, I routinely see properties that have not been decorated for 50 years. Most leases will say something like 7 years, it used to have to be more as interiors were blackened by coal fires but now it is whenever you feel like doing it.

You should paint it at least once a year

In my opinion, you should get your interior painted once every 3 years and keep changing the decor too- It keeps the freshness alive. Writings On The Wall would help you with the decor if you'd like. 

Good paint and respect for your walls depends on how long the paint will remain fresh looking. 

I have a half bathroom that is used often. It was painted about 15 years ago. I am repainting now because my taste has changed and I am redecorating. I used Pratt and Lambert paint and it still looks great, but time for a change.

Sometimes heating systems will leave dark shadows in the corners of the room or above radiators, so repainting is needed. Other ti es, children's fingerprints and scraping walls with toys will beg for a coat of paint.

Wear and tear matters on your walls. Which will determine the frequency of repainting.

Use good scrubbable paints. An eggshell finish is very easy to clean, whereas a matt finish is not. So, pick your finish wisely. The shinier the paint finish, the more imperfections will show on your walls. So, going with a satin or eggshell finish is a good choice for walls. A low gloss or gloss is good for woodwork as it is super easy to clean.

If you buy good quality paint you can go about ten years without having to paint the interior of your home and the outside too even though the rain may do some damage,  but not too much to have to repaint every three or four years.

It is not really important as far as home maintenance goes, but it's always good to have a paint color that you enjoy living with.  If you plan on selling the house, a fresh coat of paint can brighten things up and make a world of difference.  Potential buyers love freshly painted homes.

There's no real mandatory minimum, but most house paint lasts at least 10 years, 20 to 25 in good conditions, so you don't HAVE to do it very often at all, and its a massively disruptive process what with moving or covering all the furnishings so they don't get painted, so if you don't have to, its probably best avoided. 

How important it is would depend on you but it would drastically change the look of your home. It's no die-hard rule of when to repaint, so long as it's not scuffed up or holding on to odors sometimes smoke or sunlight can cause white or pale colors to yellow so you may want to repaint it then.