Chem increases at a rate of 1 per day, so that's a 0.1% increase for all attributes for every player every day.
Decline works at every 8 days, the player will lose a percentage of his stats. Strength, Speed, Agility, and Jumping all decline at the following rate:
441 days old to 480 days old (player's 12th season): 1% every 8 days (net loss of -5%)
481 days old to 520 days old (player's 13th season): 2% (net of -15%)
521 days old to 560 days old (player's 14th season): 3% (net of -30%)
561 days old to 600 days old (player's 15th season): 4% (net of -50%)
601 days old to 640 days old (player's 16th season): 5% (net of -75%)
The player's other attributes (football skills and mental abilities) decline at half the rate listed above, until they reach a net of -75% as well.
That's an old FAQ, so decline really starts at day 561. So 1% every 8 days is 0.125% a day.
So chem increase by itself pretty much balances out with decline decrease and add in additional VAs and BTs and training, the dots perform better their 1st season of decline over the course of the season as chem recovers.