How long is it between entering the seminary and diaconal ordination?
Well, first it depends on what level one enters. If one enters the seminary at the collegiate level, then it would be four years of college seminary. After that, depending on the seminary or the diocese, it's usually three or four years. In some cases, it's four years including a "pastoral year" outside the seminary in a parish setting, which is followed by deaconal ordination. In other cases, the deacon year is considered the "pastoral year."
In my own case, because the diocese for which I was ordained didn't have a seminary of it's own, I completed all four years of post-graduate formation and was put into a parish for a year, the last five months of which were spent as a deacon; so I was only a deacon for five months.