I love the idea of having art to see every day. However, Povilas S raised two salient points.
The first point is on the time frame, I think he and Juran are right on that - a night might be too short for an art piece of such scale, especially since people will be viewing it the day after.
The second point is where to display it. That was actually my first thought - "Where will it be displayed?"
In addition to the spaces he mentioned - event halls and commercial spaces - I'll like to suggest a graffiti museum and the use of sponsors.
On the use of sponsors - what if every graffiti piece is sponsored by a prominent person in the city. What's in it for them? The chance to inspire an art piece, have it named after them, or both. In return, the artist gets paid for their time and efforts.
After the piece has expended its "staying up" time, it can then be transferred to the museum. I'm undecided on whether the sponsor should be charged a "maintenance fee" for the piece or whether people who visit the museum later should be charged for that. The museum can also provide private storage for people who don't want their pieces accessible to the public.
I think this idea is great as it will motivate artists, especially young ones, to direct their energies to creative and positive ventures.
Thumps up to you, Darko Savic, for coming up with it.