How much does a front wheel last with disc brakes vs calipers? I've read that the calipers end up "eating" the rim. How many km in the city or touring (with a normal use of the brakes) does it take to destroy a rim? Is it something to worry about?

Considering that I already have calipers and I'm planning to change the wheels in the future in the range of 500€ (both)... If I change to disc brakes I would have to change the fork, buy the front disc, probably add weight...

I want to have a bike reliable with really low maintenance both in money and time which can do a lot of kms, would changing to a front disc brake save me money and time in the long term?
It will heavily depend on the conditions, but it's not something you really have to worry about. 

At a rough estimate I have 25,000 km on my hybrid, with v brakes (there shouldn't be a huge amount of difference between rim brake types). I ride in all weathers, and much of the distance is commuting with some longer road rides and dirt tracks. There's plenty of wear left in the front rim, going by the groove that's there to indicate that, and most of my braking is at the front. This is an aluminium rim, and one built more for strength the lightness.