For a DE...
Level 1-21, you gain .5 per level
Level 22-29, you gain .375 per level
Level 30-37, you gain 75% of .375 per level
etc.
For a DT...
Level 1-21, you gain (2/3) per level (.66666666666666666666666666666666666666666666666666666666666666666667)
Level 22-29, you gain .5 per level
Level 30-37, you gain .375 per level
etc.
In layman's terms, your auto level gains get cut into 75% every 8 levels starting at level 22.
Also, In your first season as a DE, Speed will get you sacks. Going straight to the 5th cap should take you around one and a half seasons (like misled said), plus you can keep EQ in Agility the whole time. Then once you start pumping Agility to the 5th cap afterwords, you'll be an absolute monster until you get out of the minors where most OTs will have decent Agility/Speed/Vision/Confidence to keep up with you, at which point you should move on to Strength. Once you 5th cap Agility, though, including equipment, you should have around 86 Speed and 102 or so Agility and you should be around level 32 or so. Then you pump Strength for a little while to keep your production up, then Tackling, then Vision, then Vision/Confidence, then SAs. This has been How to Build A DE 101.