"Seventeen months and two weeks to be exact. Obviously, I've not been counting," he says with a laugh that reveals he has somehow managed to retain a sense of humour throughout a lengthy and draining convalescence.
There is, clearly, never a good time for a player to pick up a serious injury, but in McGinn's case it seemed particularly inopportune. The then 22-year-old had established himself as a pivotal figure in Malky Mackay's Watford side when he sustained knee-ligament damage in a match against Doncaster Rovers in February 2011. The injury required surgery and the former Scotland Under-21 internationalist hasn't been seen in a Watford shirt since.
McGinn's time in rehab has coincided with significant upheaval at the Championship club. Mackay, the manager who signed him from St Mirren, left in the summer of 2011 to be replaced by Sean Dyche, his assistant, whose time in charge was so brief that McGinn never played a single game for him.
Dyche was replaced this summer by Gianfranco Zola in what was the first notable act by the club's new Italian owners. There has also been a fairly high turnover of playing staff, with the new directors bringing in a number of players from Udinese and Granada, the other clubs in their football portfolio. McGinn has observed it all from the treatment room or the gym and simply wished he could be a part of it.
That day may be finally dawning. The midfielder had thought he was fit enough to play again at the end of last season but Dyche did not want to rush his return, instead limiting the player to a handful of run-outs for the reserves and in friendlies. Zola's arrival then left McGinn having to prove himself all over again.
The Italian's coaching staff have fixed guidelines that state any player returning from injury must meet certain criteria before he can rejoin the fold. McGinn met those recently, but he is reluctant to look too far ahead, given everything that has happened. However, he hopes a return to first-team football is now not too far away.
"The past year and a half has been fairly hellish but, hopefully, I'm not that far away from getting back into it," he told Herald Sport. "I thought at the end of last season that I had finally got there as I was in the squad for the final game. I almost took it for granted that the manager would give me 10 or 15 minutes just to give me a run-out, but he decided there was no point in risking it. He wasn't expecting to get the sack so he was wanting to save me so I came back flying in pre-season but then things changed with the new manager coming in.
"It has been a real slog mentally as well as physically. Boys in the team would get minor knocks and say to me, 'I can't handle three weeks, I don't know how you can do this for 18 months'. But in the last few months the idea of playing and scoring again has been a real motivation."
McGinn's transition from primary to secondary school pupil coincided with Zola's beguiling spell as a player with Chelsea. It is hardly a surprise that the prospect of working on the training ground with the man he once tried to emulate in the playground is an enticing one.
"Who wouldn't want to play football for someone like Zola?" added McGinn. "He was a genius with a ball at his feet. The boys say in training he's still the best player and I believe it."
It may not be too long before the McGinn name is again heard in Scottish football circles. Stephen's youngest brother John is a promising 17-year-old on the cusp of making the breakthrough at St Mirren, while another sibling Paul, 21, has been training with Motherwell after being linked with Aberdeen and Dundee United.
"John is coming along well at St Mirren and has worked hard since he got an opportunity," was the fraternal assessment. "I know he's rated quite highly there so I'm sure it will work out for him. Paul has been in discussions with a few Premier League clubs so, hopefully, it won't be too long before they are playing directly against each other as one's a left-sided midfielder and the other's a right-back."
Watford, bolstered by a raft of loan signings, have made an impressive start to the new season. McGinn, though, recently signed a new one-year deal and is confident he will still have a role to play.
"I've spoken to the manager and he said he can't wait to see me out there playing," he added.
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