I think the point is that it will be safe by the time the evac command is given, the doors are opened and the slides fully inflated. No one is going to be able to give you a precise time because there are too many variables.
If your main concern is people getting sucked or blown off the slide during evac, then I can probably think of a few variables
Body mass
Position of the slide in relation to the front/back of the engine
Height of the fences on the slide
Velocity of the person going down the slide
The position of the person on the slide (middle or side)
Size of engine
Wind speed...
Engineers sometimes have to manually open the start engine valves, which involves inserting a manual crank into a hole in the engine cowling during the start process. After the start has been completed (engine confirmed at idle), the engineer moves away from the engine at right angles to the engine. As long as he/she doesn't step past a red stripe painted on the side of the nose cowling, he/she is considered safe. On a 777, the red stripe is only 1.5m (5 feet) from the inlet. When standing directly in front of the engine, there is a safety radius of 4.6m (15 feet). During winds, the margin may increase by 20%. Smaller engines, of course, have smaller safety radii. There is a much larger no-go zone behind the engine, but the risk is lower in the sense that you will only get blown about or over, not sucked into the engine.