I have always used 6 to10 sec cont/50 sec rest /10 reps per body part.
Those who favor 6 sec tend to be throwers, linemen, high jumpers in my experience.
You can tell by the drop off in contraction intensity towards the end of the rep. If it drops off much at all, I’d go to 6 sec. At 5 sec, I would imagine you could use a shorter break but maybe not as short as 25 sec if you plan on more than 10 reps.
The duration of exposure will determine the role EMS will play in the overall training scheme, so I keep the EMS blocks short to stay in the explosive area.
Better to repeat cycles at intervals rather than have one long EMS training block IMO.