Regarding how long he's been out of form for........
For those nerds who like xG stats and whatnot, these are De Gea's "xG Prevented" stats over the last few years:
19/20 (so far) - +0.60
18/19 - -3.00
17/18 - +6.70
So in season by season terms, it's pretty clear that there was a massive deterioration last season. However, if we split his generally outstanding 17/18 season in half we see a +8.50 vs -1.90 split. So saying he has been off his game for roughly a season and a half is about fair.
Though to put that in context, he still finished the 17/18 season with by far the best stats of any PL goalkeeper (the second best finishing on +3.90), largely down to his inhumanly brilliant start to the season. Whereas in 18/19...... Well, let's just say he was troubling the other end of the table.
For what it's worth, he has been 4th best in the league so far this year, albeit on a small sample size of games.