Like stated the rules give a 4 minute time.
Having put on a set of full plate before I can tell you that is a very generous time frame. But 40 minutes is a tad extreme.
The set i put on broke down to about this.
30 seconds to put on chainmail shirt
1 minute to put on sollets (boots)
1 minute to put on greaves
1 minute to put on vambraces
1 minute to put on breastplate
30 seconds to put on tasses (a belt that protects upper leg and groin area)
30 second to put on gorget
30 seconds to put on helmet
1 minute to tie on helmet
30 second to put on gauntlets
So, it takes me about 7 and half minutes to put on a full suit of plate mail if I am rushed. To put it on comfortably I would double those times.
Technically, by DnD standards I only have put on Plate Mail armor. As its is very hot in central florida I wouldn't want to actually risk fighting in Full plate armor.