First, advanced rules artillery (found in the rulebooks) or standard rules artillery (found only in errata so far)?
Standard rules artillery follow the same range rules as other weapons, unless you are using the extended/horizon range optional rule, then the max is 42", the end of long range. There are no flight times.
Advanced rules artillery says any artillery fire from on-board (both the artillery unit and target are on-board) deals damage the same turn, so you don't worry about flight time there either. You only have to calculate a flight time if the artillery (or target) is off-board.