I got a rebuilt short block, and i wanna keep it at tip top condition. right now i only have 350km and im at the start of the break in but when would you recomend water injection washing of the engine?

 

Unless your tuning and premix/OMP are way off you shouldn't be anywhere near having carbon problems.
Why not consider an inexpensive boost activated water - only auxillary injection system if your concerned about carbon. In adition to carbon control you get cooling and knock suppression with IMO, less risk than water INGESTION.
More info and reading in the Auxillary Injection section.

It also helps to post what gen. car you have.