As for duration....
How many shifts do you anticipate it will take a controller to become adept at handling maximum traffic with new system? (Largely dependent on amount/quality of SIM training beforehand?)
How many watches do you have?
What's the shift plan like?
E.g. If it is anticipated that 6 shifts is sufficient for a controller (and all controllers work 6-on/4-off (2mornings, 2afternoons, 2nights, 1sleep day, 3rest days)), using a 5-watch shift, it'd take 10days.
In practice, of course, controllers may go sick or be on annual leave so it's not quite that simple...